feat(android/playlists): stale-view snackbar + Refresh on open system playlist after rebuild

#980, parity with web e932ab43. When playlist.system_rebuilt arrives (SSE)
while a system-playlist detail screen is open, the ViewModel marks it stale and
the screen shows an indefinite "This mix was refreshed · Refresh" snackbar.
Refresh re-resolves the rotated variant via PlaylistsRepository.systemShuffle
and reuses the existing regenerated navigate-replace flow to land on the fresh
playlist id — without triggering another server rebuild (unlike the manual
regenerate button). Dismiss clears the flag. Functional behaviors were already
correct; this closes the cosmetic stale-list gap.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

android/app/src/main/java/com/fabledsword/minstrel/playlists/ui/PlaylistDetailScreen.kt

@@ -28,6 +28,10 @@ import androidx.compose.material3.IconButton
 import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.OutlinedButton
 import androidx.compose.material3.Scaffold
+import androidx.compose.material3.SnackbarDuration
+import androidx.compose.material3.SnackbarHost
+import androidx.compose.material3.SnackbarHostState
+import androidx.compose.material3.SnackbarResult
 import androidx.compose.material3.Text
 import androidx.compose.material3.TopAppBar
 import androidx.compose.runtime.Composable
@@ -139,6 +143,15 @@ class PlaylistDetailViewModel @Inject constructor(
     private val regeneratedChannel = Channel<String>(Channel.BUFFERED)
     val regenerated: Flow<String> = regeneratedChannel.receiveAsFlow()
 
+    /**
+     * #980: a system-playlist rebuild landed (SSE) while this screen is open —
+     * its list is now stale and the uuid has rotated. Drives a "this mix was
+     * refreshed → Refresh" snackbar rather than yanking the user; Refresh
+     * re-resolves the variant via [reloadRebuilt].
+     */
+    private val staleInternal = MutableStateFlow(false)
+    val stale: StateFlow<Boolean> = staleInternal.asStateFlow()
+
     val likedTrackIds: StateFlow<Set<String>> =
         likes.observeLikedTrackIds()
             .stateIn(
@@ -163,6 +176,12 @@ class PlaylistDetailViewModel @Inject constructor(
      * ignored — the playlists-list screen handles those.
      */
     private fun handlePlaylistEvent(event: LiveEvent) {
+        // #980: a system rebuild carries no playlist_id (it rebuilds all of the
+        // user's system mixes at once) — handle it before the per-id filter.
+        if (event.kind == "playlist.system_rebuilt") {
+            markStaleIfSystem()
+            return
+        }
         val eventPlaylistId = event.data["playlist_id"]?.jsonPrimitive?.contentOrNull
         if (eventPlaylistId != playlistId) return
         when (event.kind) {
@@ -171,6 +190,14 @@ class PlaylistDetailViewModel @Inject constructor(
         }
     }
 
+    private fun markStaleIfSystem() {
+        val playlist = (internal.value as? PlaylistDetailUiState.Success)
+            ?.detail?.playlist ?: return
+        if (playlist.systemVariant?.takeIf { playlist.refreshable } != null) {
+            staleInternal.value = true
+        }
+    }
+
     fun toggleLikeTrack(trackId: String) {
         val desired = trackId !in likedTrackIds.value
         viewModelScope.launch {
@@ -220,6 +247,30 @@ class PlaylistDetailViewModel @Inject constructor(
         }
     }
 
+    /**
+     * #980: user tapped Refresh on the stale-view snackbar. The rebuild
+     * already ran server-side (uuid rotated) — re-resolve the variant to the
+     * fresh playlist id and hand it to [regenerated] for navigate-replace.
+     * Unlike [regenerate] this does NOT trigger another rebuild.
+     */
+    fun reloadRebuilt() {
+        val playlist = (internal.value as? PlaylistDetailUiState.Success)
+            ?.detail?.playlist ?: return
+        val variant = playlist.systemVariant?.takeIf { playlist.refreshable } ?: return
+        staleInternal.value = false
+        viewModelScope.launch {
+            runCatching { repository.systemShuffle(variant).playlist.id }
+                .onSuccess { newId ->
+                    if (newId.isNotEmpty()) regeneratedChannel.trySend(newId)
+                }
+        }
+    }
+
+    /** #980: user dismissed the stale-view snackbar without refreshing. */
+    fun dismissStale() {
+        staleInternal.value = false
+    }
+
     /** Play the available tracks starting at [startTrackId] (or first available). */
     fun play(tracks: List<PlaylistTrackRef>, startTrackId: String?) {
         val refs = tracks.toPlayableTrackRefs()
@@ -261,8 +312,25 @@ fun PlaylistDetailScreen(
             }
         }
     }
+    // #980: a rebuild landed while this system-playlist screen is open. Offer a
+    // non-intrusive "refreshed → Refresh" snackbar; Refresh re-resolves the
+    // rotated uuid (via the regenerated navigate-replace flow).
+    val snackbarHostState = remember { SnackbarHostState() }
+    val stale by viewModel.stale.collectAsState()
+    LaunchedEffect(stale) {
+        if (stale) {
+            val result = snackbarHostState.showSnackbar(
+                message = "This mix was refreshed",
+                actionLabel = "Refresh",
+                duration = SnackbarDuration.Indefinite,
+            )
+            if (result == SnackbarResult.ActionPerformed) viewModel.reloadRebuilt()
+            else viewModel.dismissStale()
+        }
+    }
     Scaffold(
         modifier = Modifier.fillMaxSize(),
+        snackbarHost = { SnackbarHost(snackbarHostState) },
         topBar = {
             TopAppBar(
                 title = { Text(currentTitle(state)) },