feat(queue): backend queue state machine replacing frontend shuffle, navigation, and play-next logic (#42)

* feat(queue): backend queue state machine replacing frontend shuffle, navigation, and play-next logic

Move ~500 lines of queue state machine from JS frontend to Rust backend.
Frontend queue store becomes a thin reactive layer calling backend commands
and applying returned state snapshots.

Backend:
- Extend queue_state table with play_next_offset, play_history_json,
  play_next_track_ids_json, repeat_one_pending columns
- Add toggle_shuffle (Fisher-Yates with play-next pinning),
  add_play_next (move semantics + offset tracking),
  advance_to_next/previous (repeat-one two-phase, loop modes, history),
  skip_to_next/previous (override repeat-one), check_integrity
- Register 6 new Tauri commands, enhance queue_set_shuffle to return
  QueueStateSnapshot
- 773 Rust tests pass

Frontend:
- Remove _shuffleItems, _reshuffleForLoopRestart, _originalOrder,
  _playHistory, _playNextTrackIds, _playNextOffset, _repeatOnePending,
  _syncQueueToBackend, _validateQueueIntegrity
- Add _applySnapshot and _applyNavigationResult
- Update queue-builder and queue.props tests for backend-delegated pattern
- 444 Vitest tests pass

Closes TASK-328

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix(queue): resolve deno lint errors in queue store

Remove unnecessary async from _applyNavigationResult (no await inside).
Prefix unused fromNavigation parameter with underscore.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* fix(lint): collapse nested if in watcher scan completion

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* fix(queue): optimistic shuffle toggle and aria-pressed for accessibility

toggleShuffle now sets this.shuffle before the backend call (like
cycleLoop does for loop mode), reverting on failure. Add aria-pressed
to the shuffle button so the E2E accessibility test detects state change.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: pythoninthegrass

app/frontend/__tests__/queue-builder.test.js

@@ -46,11 +46,6 @@ function createMockCtx(queueOverrides = {}, playerOverrides = {}) {
       currentIndex: -1,
       shuffle: false,
       _updating: false,
-      _originalOrder: [],
-      _playHistory: [],
-      _playNextOffset: 0,
-      _playNextTrackIds: new Set(),
-      _pushToHistory: vi.fn(),
       ...queueOverrides,
     },
     player: {
@@ -131,37 +126,17 @@ describe('handleDoubleClickPlay', () => {
     expect(ctx.queue.currentIndex).toBe(1);
   });
 
-  it('resets queue state fields after play context', async () => {
+  it('applies shuffle_enabled from result', async () => {
     const tracks = makeTracks(['A', 'B']);
     const result = makePlayContextResult(tracks, 0);
+    result.shuffle_enabled = true;
     queueApi.playContext.mockResolvedValue(result);
 
-    const existingHistory = [{ id: 99, title: 'Old' }];
-    const ctx = createMockCtx({
-      _playHistory: existingHistory,
-      _playNextOffset: 5,
-      _playNextTrackIds: new Set([99]),
-    });
-
-    await handleDoubleClickPlay(ctx, tracks[0], tracks, 0, 'test');
-
-    expect(ctx.queue._playHistory).toEqual([]);
-    expect(ctx.queue._playNextOffset).toBe(0);
-    expect(ctx.queue._playNextTrackIds.size).toBe(0);
-  });
-
-  it('sets _originalOrder to match items', async () => {
-    const tracks = makeTracks(['A', 'B', 'C']);
-    const result = makePlayContextResult(tracks, 0);
-    queueApi.playContext.mockResolvedValue(result);
-
-    const ctx = createMockCtx();
+    const ctx = createMockCtx({ shuffle: false });
 
     await handleDoubleClickPlay(ctx, tracks[0], tracks, 0, 'test');
 
-    expect(ctx.queue._originalOrder).toEqual(tracks);
-    // Should be a copy, not the same reference
-    expect(ctx.queue._originalOrder).not.toBe(ctx.queue.items);
+    expect(ctx.queue.shuffle).toBe(true);
   });
 
   it('calls updateTrackState with track and duration_ms', async () => {