feat(android): MiniPlayer → NowPlaying cover Hero transition (audit v2 #17, slice 2)

Wraps the NavHost in a SharedTransitionLayout and applies
Modifier.sharedElement to both the MiniPlayer cover and the
NowPlayingCover, keyed on a single HERO_KEY_NOW_PLAYING_COVER.
Tapping the MiniPlayer cover now morphs into the full NowPlaying
cover instead of cross-fading.

Plumbing:
* HeroScopes.kt — staticCompositionLocalOf holders for both the
  SharedTransitionScope (set once at NavHost root) and the
  AnimatedContentScope (re-set per composable<>, since each route
  has its own).
* MinstrelNavGraph.kt — private WithAnimatedScope helper wraps
  each composable<> lambda so its AnimatedContentScope reaches
  the nested cover.
* MiniPlayer.kt MiniCover + NowPlayingScreen.kt NowPlayingCover
  each read both scopes and prepend Modifier.sharedElement when
  both are present; degrade gracefully (no hero, still renders)
  outside the layout for previews / tests.

Cover preload still pending — that's slice 3 if you want it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

android/app/src/main/java/com/fabledsword/minstrel/nav/HeroScopes.kt

@@ -0,0 +1,27 @@
+package com.fabledsword.minstrel.nav
+
+import androidx.compose.animation.AnimatedContentScope
+import androidx.compose.animation.ExperimentalSharedTransitionApi
+import androidx.compose.animation.SharedTransitionScope
+import androidx.compose.runtime.staticCompositionLocalOf
+
+/**
+ * Scope hand-off for shared-element (Hero) transitions between
+ * MiniPlayer and NowPlaying covers.
+ *
+ * [LocalSharedTransitionScope] is provided once by the
+ * `SharedTransitionLayout` wrapping the NavHost.
+ * [LocalAnimatedContentScope] is re-provided per `composable<X>`
+ * destination since each route has its own AnimatedContentScope.
+ *
+ * Both are nullable so widgets degrade gracefully (no hero, but
+ * still render) outside a SharedTransitionLayout — useful in
+ * previews and tests.
+ */
+@OptIn(ExperimentalSharedTransitionApi::class)
+val LocalSharedTransitionScope = staticCompositionLocalOf<SharedTransitionScope?> { null }
+
+val LocalAnimatedContentScope = staticCompositionLocalOf<AnimatedContentScope?> { null }
+
+/** Shared-element key for the MiniPlayer → NowPlaying cover hero. */
+const val HERO_KEY_NOW_PLAYING_COVER = "now-playing-cover"

android/app/src/main/java/com/fabledsword/minstrel/nav/MinstrelNavGraph.kt

@@ -1,9 +1,13 @@
 package com.fabledsword.minstrel.nav
 
+import androidx.compose.animation.AnimatedContentScope
+import androidx.compose.animation.ExperimentalSharedTransitionApi
+import androidx.compose.animation.SharedTransitionLayout
 import androidx.compose.animation.slideInVertically
 import androidx.compose.animation.slideOutVertically
 import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.runtime.Composable
+import androidx.compose.runtime.CompositionLocalProvider
 import androidx.compose.ui.Modifier
 import androidx.navigation.NavGraphBuilder
 import androidx.navigation.NavHostController
@@ -37,6 +41,7 @@ import com.fabledsword.minstrel.shared.widgets.ShellScaffold
  * Per-route transition: only NowPlaying gets the modal slide-up; the
  * rest use the default fade.
  */
+@OptIn(ExperimentalSharedTransitionApi::class)
 @Composable
 fun MinstrelNavGraph(
     navController: NavHostController,
@@ -44,14 +49,18 @@ fun MinstrelNavGraph(
     modifier: Modifier = Modifier,
 ) {
     val expandPlayer = { navController.navigate(NowPlaying) }
-    NavHost(
-        navController = navController,
-        startDestination = startDestination,
-        modifier = modifier.fillMaxSize(),
-    ) {
-        inShellTopLevel(navController, expandPlayer)
-        inShellDetail(navController, expandPlayer)
-        outsideShell(navController)
+    SharedTransitionLayout(modifier = modifier.fillMaxSize()) {
+        CompositionLocalProvider(LocalSharedTransitionScope provides this) {
+            NavHost(
+                navController = navController,
+                startDestination = startDestination,
+                modifier = Modifier.fillMaxSize(),
+            ) {
+                inShellTopLevel(navController, expandPlayer)
+                inShellDetail(navController, expandPlayer)
+                outsideShell(navController)
+            }
+        }
     }
 }
 
@@ -60,79 +69,125 @@ private fun NavGraphBuilder.inShellTopLevel(
     expandPlayer: () -> Unit,
 ) {
     composable<Home> {
-        ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
-            HomeScreen(navController = navController)
+        WithAnimatedScope {
+            ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
+                HomeScreen(navController = navController)
+            }
         }
     }
     composable<Library> {
-        ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
-            LibraryScreen(navController = navController)
+        WithAnimatedScope {
+            ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
+                LibraryScreen(navController = navController)
+            }
         }
     }
     composable<Search> {
-        ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
-            SearchScreen(navController = navController)
+        WithAnimatedScope {
+            ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
+                SearchScreen(navController = navController)
+            }
         }
     }
     composable<Discover> {
-        ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
-            DiscoverScreen(navController = navController)
+        WithAnimatedScope {
+            ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
+                DiscoverScreen(navController = navController)
+            }
         }
     }
     composable<Playlists> {
-        ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
-            PlaylistsListScreen(navController = navController)
+        WithAnimatedScope {
+            ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
+                PlaylistsListScreen(navController = navController)
+            }
         }
     }
     composable<Settings> {
-        ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
-            SettingsScreen(navController = navController)
+        WithAnimatedScope {
+            ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
+                SettingsScreen(navController = navController)
+            }
         }
     }
     composable<Admin> {
-        ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
-            AdminLandingScreen(navController = navController)
+        WithAnimatedScope {
+            ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
+                AdminLandingScreen(navController = navController)
+            }
         }
     }
     composable<Requests> {
-        ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
-            RequestsScreen(navController = navController)
+        WithAnimatedScope {
+            ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
+                RequestsScreen(navController = navController)
+            }
         }
     }
 }
 
+/**
+ * Wraps content with [LocalAnimatedContentScope] sourced from the
+ * enclosing `composable<>` lambda's [AnimatedContentScope] receiver
+ * so the MiniPlayer / NowPlaying covers can apply
+ * `Modifier.sharedElement(...)` for the Hero transition.
+ *
+ * Must be invoked from inside a `composable<>` lambda — its `this`
+ * is the [AnimatedContentScope].
+ */
+@Composable
+private fun AnimatedContentScope.WithAnimatedScope(
+    content: @Composable () -> Unit,
+) {
+    CompositionLocalProvider(LocalAnimatedContentScope provides this) {
+        content()
+    }
+}
+
 private fun NavGraphBuilder.inShellDetail(
     navController: NavHostController,
     expandPlayer: () -> Unit,
 ) {
     composable<AlbumDetail> {
-        ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
-            AlbumDetailScreen(navController = navController)
+        WithAnimatedScope {
+            ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
+                AlbumDetailScreen(navController = navController)
+            }
         }
     }
     composable<ArtistDetail> {
-        ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
-            ArtistDetailScreen(navController = navController)
+        WithAnimatedScope {
+            ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
+                ArtistDetailScreen(navController = navController)
+            }
         }
     }
     composable<PlaylistDetail> {
-        ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
-            PlaylistDetailScreen(navController = navController)
+        WithAnimatedScope {
+            ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
+                PlaylistDetailScreen(navController = navController)
+            }
         }
     }
     composable<AdminRequests> {
-        ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
-            AdminRequestsScreen(navController = navController)
+        WithAnimatedScope {
+            ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
+                AdminRequestsScreen(navController = navController)
+            }
         }
     }
     composable<AdminQuarantine> {
-        ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
-            AdminQuarantineScreen(navController = navController)
+        WithAnimatedScope {
+            ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
+                AdminQuarantineScreen(navController = navController)
+            }
         }
     }
     composable<AdminUsers> {
-        ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
-            AdminUsersScreen(navController = navController)
+        WithAnimatedScope {
+            ShellScaffold(navController = navController, onExpandPlayer = expandPlayer) {
+                AdminUsersScreen(navController = navController)
+            }
         }
     }
 }
@@ -146,7 +201,9 @@ private fun NavGraphBuilder.outsideShell(navController: NavHostController) {
         popEnterTransition = { slideInVertically(initialOffsetY = { it }) },
         popExitTransition = { slideOutVertically(targetOffsetY = { it }) },
     ) {
-        NowPlayingScreen(navController = navController)
+        WithAnimatedScope {
+            NowPlayingScreen(navController = navController)
+        }
     }
     composable<Queue> { QueueScreen(navController = navController) }
     composable<ServerUrl> { ServerUrlScreen(navController = navController) }

android/app/src/main/java/com/fabledsword/minstrel/player/ui/MiniPlayer.kt

@@ -1,5 +1,6 @@
 package com.fabledsword.minstrel.player.ui
 
+import androidx.compose.animation.ExperimentalSharedTransitionApi
 import androidx.compose.foundation.background
 import androidx.compose.foundation.clickable
 import androidx.compose.foundation.layout.Box
@@ -37,6 +38,9 @@ import com.composables.icons.lucide.Play
 import com.composables.icons.lucide.SkipBack
 import com.composables.icons.lucide.SkipForward
 import com.fabledsword.minstrel.models.TrackRef
+import com.fabledsword.minstrel.nav.HERO_KEY_NOW_PLAYING_COVER
+import com.fabledsword.minstrel.nav.LocalAnimatedContentScope
+import com.fabledsword.minstrel.nav.LocalSharedTransitionScope
 import com.fabledsword.minstrel.shared.widgets.LikeButton
 import com.fabledsword.minstrel.shared.widgets.trackactions.TrackActionsButton
 import com.fabledsword.minstrel.shared.widgets.trackactions.TrackActionsViewModel
@@ -47,10 +51,25 @@ private const val MINI_BAR_TONAL_ELEVATION_DP = 4
 private const val COVER_SIZE_DP = 48
 private const val SCRUBBER_ROW_HEIGHT_DP = 4
 
+@OptIn(ExperimentalSharedTransitionApi::class)
 @Composable
 private fun MiniCover(coverUrl: String, contentDescription: String) {
+    val sharedScope = LocalSharedTransitionScope.current
+    val animatedScope = LocalAnimatedContentScope.current
+    val heroModifier = if (sharedScope != null && animatedScope != null) {
+        with(sharedScope) {
+            Modifier.sharedElement(
+                sharedContentState = rememberSharedContentState(
+                    key = HERO_KEY_NOW_PLAYING_COVER,
+                ),
+                animatedVisibilityScope = animatedScope,
+            )
+        }
+    } else {
+        Modifier
+    }
     Box(
-        modifier = Modifier
+        modifier = heroModifier
             .size(COVER_SIZE_DP.dp)
             .clip(RoundedCornerShape(FabledSwordFlatTokens.radiusSm))
             .background(MaterialTheme.colorScheme.surfaceVariant),

android/app/src/main/java/com/fabledsword/minstrel/player/ui/NowPlayingScreen.kt

@@ -2,6 +2,7 @@
 
 package com.fabledsword.minstrel.player.ui
 
+import androidx.compose.animation.ExperimentalSharedTransitionApi
 import androidx.compose.foundation.background
 import androidx.compose.foundation.layout.Arrangement
 import androidx.compose.foundation.layout.Box
@@ -61,6 +62,9 @@ import com.composables.icons.lucide.SkipForward
 import com.fabledsword.minstrel.player.RepeatMode
 import com.fabledsword.minstrel.nav.AlbumDetail
 import com.fabledsword.minstrel.nav.ArtistDetail
+import com.fabledsword.minstrel.nav.HERO_KEY_NOW_PLAYING_COVER
+import com.fabledsword.minstrel.nav.LocalAnimatedContentScope
+import com.fabledsword.minstrel.nav.LocalSharedTransitionScope
 import com.fabledsword.minstrel.nav.Queue
 import com.fabledsword.minstrel.shared.widgets.trackactions.TrackActionsButton
 import com.fabledsword.minstrel.shared.widgets.trackactions.TrackActionsViewModel
@@ -299,10 +303,25 @@ private fun ViewQueueButton(onClick: () -> Unit) {
     }
 }
 
+@OptIn(ExperimentalSharedTransitionApi::class)
 @Composable
 private fun NowPlayingCover(coverUrl: String, contentDescription: String) {
+    val sharedScope = LocalSharedTransitionScope.current
+    val animatedScope = LocalAnimatedContentScope.current
+    val heroModifier = if (sharedScope != null && animatedScope != null) {
+        with(sharedScope) {
+            Modifier.sharedElement(
+                sharedContentState = rememberSharedContentState(
+                    key = HERO_KEY_NOW_PLAYING_COVER,
+                ),
+                animatedVisibilityScope = animatedScope,
+            )
+        }
+    } else {
+        Modifier
+    }
     Box(
-        modifier = Modifier
+        modifier = heroModifier
             .widthIn(max = COVER_MAX_WIDTH_DP.dp)
             .fillMaxWidth()
             .aspectRatio(1f)