feat(android): NowPlaying dominant-color gradient backdrop (audit v2 #17, slice 1)

Adds androidx.palette dependency and a rememberDominantColor()
helper that pulls the cover bitmap via Coil's singleton loader
(shares the cache with the on-screen AsyncImage), runs Palette
extraction on Dispatchers.Default, and animates the resulting
color with animateColorAsState so track changes tween smoothly.

NowPlayingScreen wraps the body in a Box with a vertical gradient
(0% dominant @ 55%α → 45% dominant @ 18%α → 100% scheme.background)
and lets the Scaffold's containerColor go transparent so the
gradient shows through. Falls back to a near-transparent gradient
on bitmap-load failure so the screen never sits on flat black.

Hero transition (MiniPlayer → NowPlaying cover) and cover
preload still pending — those land in a follow-up commit.

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

android/app/build.gradle.kts

@@ -165,6 +165,7 @@ dependencies {
 
     implementation(libs.coil.compose)
     implementation(libs.coil.network.okhttp)
+    implementation(libs.androidx.palette)
     implementation(libs.icons.lucide)
 
     implementation(libs.timber)

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

@@ -0,0 +1,69 @@
+package com.fabledsword.minstrel.player.ui
+
+import androidx.compose.animation.animateColorAsState
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
+import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
+import androidx.compose.ui.graphics.Color
+import androidx.compose.ui.platform.LocalContext
+import androidx.palette.graphics.Palette
+import coil3.SingletonImageLoader
+import coil3.request.ImageRequest
+import coil3.request.SuccessResult
+import coil3.request.allowHardware
+import coil3.toBitmap
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.withContext
+
+private const val GRADIENT_TWEEN_MS = 600
+
+/**
+ * Animated dominant-color extractor for the NowPlaying gradient
+ * backdrop. Fetches [coverUrl] via Coil's singleton loader (so the
+ * bitmap is shared with the on-screen AsyncImage cache), runs
+ * androidx.palette on a background thread, and animates the result
+ * via [animateColorAsState] so track changes tween smoothly.
+ *
+ * Returns [Color.Transparent] while the bitmap is loading or on
+ * any failure — callers should layer a fallback (e.g.
+ * `MaterialTheme.colorScheme.surface`) underneath the gradient
+ * tint so the screen never sits on a flat black.
+ */
+@Composable
+fun rememberDominantColor(coverUrl: String?): Color {
+    val context = LocalContext.current
+    var extracted by remember(coverUrl) { mutableStateOf(Color.Transparent) }
+
+    LaunchedEffect(coverUrl) {
+        if (coverUrl.isNullOrEmpty()) {
+            extracted = Color.Transparent
+            return@LaunchedEffect
+        }
+        val request = ImageRequest.Builder(context)
+            .data(coverUrl)
+            .allowHardware(false)
+            .build()
+        val result = SingletonImageLoader.get(context).execute(request)
+        if (result !is SuccessResult) return@LaunchedEffect
+        val bitmap = result.image.toBitmap()
+        val palette = withContext(Dispatchers.Default) {
+            Palette.from(bitmap).generate()
+        }
+        val swatch = palette.vibrantSwatch
+            ?: palette.mutedSwatch
+            ?: palette.dominantSwatch
+        if (swatch != null) {
+            extracted = Color(swatch.rgb)
+        }
+    }
+
+    val animated by animateColorAsState(
+        targetValue = extracted,
+        animationSpec = androidx.compose.animation.core.tween(GRADIENT_TWEEN_MS),
+        label = "now-playing-dominant",
+    )
+    return animated
+}

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

@@ -1,3 +1,5 @@
+@file:Suppress("TooManyFunctions") // Compose screen + private helper composables
+
 package com.fabledsword.minstrel.player.ui
 
 import androidx.compose.foundation.background
@@ -23,6 +25,7 @@ import androidx.compose.material3.Slider
 import androidx.compose.material3.TopAppBar
 import androidx.compose.material3.TopAppBarDefaults
 import androidx.compose.material3.ExperimentalMaterial3Api
+import androidx.compose.ui.graphics.Brush
 import androidx.compose.ui.graphics.Color
 import androidx.compose.ui.input.pointer.pointerInput
 import androidx.compose.material3.SliderDefaults
@@ -111,22 +114,43 @@ fun NowPlayingScreen(
         }
         return
     }
+    val dominant = rememberDominantColor(track.coverUrl)
     Scaffold(
         modifier = Modifier.fillMaxSize().nowPlayingDragDismiss(navController),
         topBar = { NowPlayingTopBar(onClose = { navController.popBackStack() }) },
         snackbarHost = { SnackbarHost(snackbarHostState) },
-        containerColor = MaterialTheme.colorScheme.background,
+        containerColor = Color.Transparent,
     ) { inner ->
-        NowPlayingBody(
-            inner = inner,
-            state = state,
-            track = track,
-            navController = navController,
-            viewModel = viewModel,
-        )
+        Box(
+            modifier = Modifier
+                .fillMaxSize()
+                .background(dominantGradient(dominant)),
+        ) {
+            NowPlayingBody(
+                inner = inner,
+                state = state,
+                track = track,
+                navController = navController,
+                viewModel = viewModel,
+            )
+        }
     }
 }
 
+@Composable
+private fun dominantGradient(top: Color): Brush {
+    val base = MaterialTheme.colorScheme.background
+    return Brush.verticalGradient(
+        0f to top.copy(alpha = DOMINANT_TINT_TOP),
+        DOMINANT_TINT_MID_STOP to top.copy(alpha = DOMINANT_TINT_MID),
+        1f to base,
+    )
+}
+
+private const val DOMINANT_TINT_TOP = 0.55f
+private const val DOMINANT_TINT_MID = 0.18f
+private const val DOMINANT_TINT_MID_STOP = 0.45f
+
 /**
  * Drag-down to dismiss. Mirrors Flutter's modal-page gesture: a
  * downward drag past the threshold pops the screen, returning the

android/gradle/libs.versions.toml

@@ -23,6 +23,7 @@ kotlinx-datetime = "0.6.1"
 kotlinx-serialization-converter = "1.0.0"
 media3 = "1.10.1"
 coil = "3.0.0-rc02"
+palette = "1.0.0"
 timber = "5.0.1"
 work = "2.10.0"
 lifecycle = "2.8.7"
@@ -72,6 +73,7 @@ media3-datasource-okhttp = { module = "androidx.media3:media3-datasource-okhttp"
 media3-ui = { module = "androidx.media3:media3-ui", version.ref = "media3" }
 coil-compose = { module = "io.coil-kt.coil3:coil-compose", version.ref = "coil" }
 coil-network-okhttp = { module = "io.coil-kt.coil3:coil-network-okhttp", version.ref = "coil" }
+androidx-palette = { module = "androidx.palette:palette-ktx", version.ref = "palette" }
 # CMP (Compose Multiplatform) variant — provides icons as ImageVector
 # objects (e.g. `Icon(Lucide.Disc3, ...)`). The -android variant ships
 # Vector Drawable XML accessed via painterResource(), which is less