fix(playlists): surface playlist-load failures with SnackBar feedback (#479)

Tapping a system-playlist PlayCircleButton before the mix had loaded
silently stalled — three failure modes all looked identical to the
user (PlayCircleButton's spinner clears with no playback):

  - api.systemShuffle returns empty (mix not built server-side yet)
    → silent return at `if (refs.isEmpty)`
  - api call slow / hung → spinner spins indefinitely (no client
    timeout was set; Dio default is unlimited)
  - api throws → uncaught; spinner finalizer clears it silently

Bundle:

  - 8s `.timeout` on the systemShuffle / get call; TimeoutException
    → "Couldn't load playlist — check your connection"
  - empty refs after filtering → "Mix isn't ready yet — try again
    in a moment"
  - other throws → "Playlist load failed: <error>"
  - thread BuildContext through and capture ScaffoldMessenger before
    the first await so no `use_build_context_synchronously` lint

No pre-warm — system playlists are intentionally uncached per the
api endpoint comment ("varies per play"); pre-warming would decide
the shuffle order at home-screen load instead of at tap, breaking
the fresh-per-play contract.

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

flutter_client/lib/playlists/widgets/playlist_card.dart

@@ -1,3 +1,5 @@
+import 'dart:async';
+
 import 'package:flutter/material.dart';
 import 'package:flutter_lucide/flutter_lucide.dart';
 import 'package:flutter_riverpod/flutter_riverpod.dart';
@@ -70,7 +72,7 @@ class PlaylistCard extends ConsumerWidget {
                     right: 6,
                     child: PlayCircleButton(
                       enabled: hasTracks,
-                      onPressed: () => _playPlaylist(ref),
+                      onPressed: () => _playPlaylist(context, ref),
                     ),
                   ),
                   // System playlists get a refresh affordance so the
@@ -187,16 +189,34 @@ class PlaylistCard extends ConsumerWidget {
   /// PlaylistTrack into a TrackRef (filtering out unavailable rows
   /// whose `trackId` is null after a track-delete), and plays from
   /// index 0. Mirrors the web PlaylistCard's onPlayClick.
-  Future<void> _playPlaylist(WidgetRef ref) async {
+  ///
+  /// Failure modes are surfaced via SnackBar so a not-yet-built mix
+  /// or a slow server doesn't look like the tap was ignored.
+  Future<void> _playPlaylist(BuildContext context, WidgetRef ref) async {
+    final messenger = ScaffoldMessenger.of(context);
     final api = await ref.read(playlistsApiProvider.future);
-    // Refreshable (singleton) system playlists: fetch the server's
+    final PlaylistDetail detail;
-    // rotation-aware order (#415) and play as-is, tagged with the
+    try {
-    // source so the reporter advances rotation. User playlists AND
+      // Refreshable (singleton) system playlists: fetch the server's
-    // non-singleton system kinds (songs_like_artist — no by-kind
+      // rotation-aware order (#415) and play as-is, tagged with the
-    // endpoint) play the stored order via get(), untagged.
+      // source so the reporter advances rotation. User playlists AND
-    final detail = playlist.refreshable
+      // non-singleton system kinds (songs_like_artist — no by-kind
-        ? await api.systemShuffle(playlist.systemVariant!)
+      // endpoint) play the stored order via get(), untagged.
-        : await api.get(playlist.id);
+      detail = await (playlist.refreshable
+              ? api.systemShuffle(playlist.systemVariant!)
+              : api.get(playlist.id))
+          .timeout(const Duration(seconds: 8));
+    } on TimeoutException {
+      messenger.showSnackBar(const SnackBar(
+        content: Text("Couldn't load playlist — check your connection"),
+      ));
+      return;
+    } catch (e) {
+      messenger.showSnackBar(SnackBar(
+        content: Text('Playlist load failed: $e'),
+      ));
+      return;
+    }
     final refs = <TrackRef>[];
     for (final t in detail.tracks) {
       if (t.trackId == null) continue;
@@ -211,7 +231,12 @@ class PlaylistCard extends ConsumerWidget {
         streamUrl: t.streamUrl ?? '',
       ));
     }
-    if (refs.isEmpty) return;
+    if (refs.isEmpty) {
+      messenger.showSnackBar(const SnackBar(
+        content: Text("Mix isn't ready yet — try again in a moment"),
+      ));
+      return;
+    }
     // Rotation-aware kinds arrive pre-ordered from the server — play
     // as-is, tagged so the reporter advances rotation. No client
     // shuffle. Everything else plays stored order, untagged.