feat(player): resume last session on launch (#54)

The #52 teardown clears the session when idle/dismissed, so the
headset / lock-screen play button had nothing to resume and the user
lost their place.

- track.dart: toJson() (round-trips fromJson)
- db.dart: CachedResumeState single-row snapshot, schema 9->10 + migration
- audio_handler.dart: queuedTracks getter
- player_provider.dart: restoreQueue() — configure + setQueueFromTracks
  + seek, no play (restores PAUSED)
- resume_controller.dart: restores last snapshot paused on launch;
  persists {source,index,position_ms,tracks} debounced on track
  change / pause and immediately on app teardown; never clobbers the
  saved snapshot when the queue is empty so a teardown stays
  recoverable; restore gated on auth + no active session
- app.dart: wired into postFrame

db.g.dart regenerated by CI per project convention. Deferred fast-follow:
media-button-when-fully-stopped re-init (needs a configure()-injected
callback; tracked on #54).

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

flutter_client/lib/cache/db.dart

@@ -230,6 +230,20 @@ class CachedQuarantineMine extends Table {
   Set<Column> get primaryKey => {trackId};
 }
 
+/// Single-row snapshot of the last playback session — queue (TrackRef
+/// JSON), current index, position, and #415 source. Lets a torn-down
+/// session (the #52 idle/dismissed teardown) resume on next launch;
+/// without it the headset / lock-screen play button has nothing to
+/// resume. Mirrors the CachedHomeSnapshot single-row JSON pattern.
+/// Schema 10+.
+class CachedResumeState extends Table {
+  IntColumn get id => integer().withDefault(const Constant(1))();
+  TextColumn get json => text()();
+  DateTimeColumn get updatedAt => dateTime().withDefault(currentDateAndTime)();
+  @override
+  Set<Column> get primaryKey => {id};
+}
+
 enum CacheSource { manual, autoLiked, autoPlaylist, autoPrefetch, incidental }
 
 @DriftDatabase(tables: [
@@ -247,12 +261,13 @@ enum CacheSource { manual, autoLiked, autoPlaylist, autoPrefetch, incidental }
   CachedHomeIndex,
   CachedSystemPlaylistsStatus,
   CachedMutations,
+  CachedResumeState,
 ])
 class AppDb extends _$AppDb {
   AppDb([QueryExecutor? e]) : super(e ?? driftDatabase(name: 'minstrel_cache'));
 
   @override
-  int get schemaVersion => 9;
+  int get schemaVersion => 10;
 
   @override
   MigrationStrategy get migration => MigrationStrategy(
@@ -316,6 +331,12 @@ class AppDb extends _$AppDb {
               'UPDATE audio_cache_index SET last_played_at = cached_at',
             );
           }
+          if (from < 10) {
+            // Schema 10 (#54): cached_resume_state — single-row last-
+            // session snapshot for resume-on-launch. Empty on upgrade;
+            // first persist populates it.
+            await m.createTable(cachedResumeState);
+          }
         },
       );
 }