feat(#119): saved queues web UI + behavior-settings toggles

- Settings → 行为设置: two new toggles (保存/加载播放清单, 单曲直接播放不清空队列)
  that round-trip savedQueuesEnabled / playKeepsQueue and keep the nav gate in sync.
- New "已存队列" page (/saved-queues): save the current queue (with a 共享 option),
  load (replace) / append / delete saved queues; renders a "feature disabled"
  state on 403 so it degrades gracefully when the flag is off.
- Nav entry gated on the savedQueuesEnabled store flag (hidden for guests).
- useSavedQueues API composable + a pure, unit-tested list/ownership helper.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

web/src/composables/savedQueues.test.ts

@@ -0,0 +1,34 @@
+import { describe, it, expect } from 'vitest';
+import { isShared, sortQueues, SHARED_OWNER, type SavedQueueMeta } from './savedQueues';
+
+const meta = (over: Partial<SavedQueueMeta>): SavedQueueMeta => ({
+  id: 1,
+  ownerId: 'u1',
+  name: 'q',
+  songCount: 0,
+  createdAt: '2026-01-01',
+  updatedAt: '2026-01-01',
+  ...over,
+});
+
+describe('savedQueues helper', () => {
+  it('flags the shared owner', () => {
+    expect(isShared({ ownerId: SHARED_OWNER })).toBe(true);
+    expect(isShared({ ownerId: 'u1' })).toBe(false);
+  });
+
+  it('sorts by updatedAt descending (newest first)', () => {
+    const out = sortQueues([
+      meta({ id: 1, updatedAt: '2026-01-01' }),
+      meta({ id: 2, updatedAt: '2026-02-01' }),
+      meta({ id: 3, updatedAt: '2026-01-15' }),
+    ]);
+    expect(out.map((q) => q.id)).toEqual([2, 3, 1]);
+  });
+
+  it('does not mutate the input array', () => {
+    const input = [meta({ id: 1, updatedAt: '2026-01-01' }), meta({ id: 2, updatedAt: '2026-02-01' })];
+    sortQueues(input);
+    expect(input.map((q) => q.id)).toEqual([1, 2]);
+  });
+});