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.ts

@@ -0,0 +1,27 @@
+// Pure, framework-free helpers for the Saved Queues feature (#119). Kept
+// separate from the API composable so the list/ownership logic is unit-testable
+// without a DOM or axios.
+
+export interface SavedQueueMeta {
+  id: number;
+  ownerId: string;
+  name: string;
+  songCount: number;
+  createdAt: string;
+  updatedAt: string;
+}
+
+/** Must match SHARED_QUEUE_OWNER in src/data/database.ts. */
+export const SHARED_OWNER = '__shared__';
+
+/** Whether a saved queue lives in the shared bucket (vs. private to a user). */
+export function isShared(q: Pick<SavedQueueMeta, 'ownerId'>): boolean {
+  return q.ownerId === SHARED_OWNER;
+}
+
+/** Newest-updated first; stable for equal timestamps. */
+export function sortQueues<T extends { updatedAt: string }>(qs: T[]): T[] {
+  return [...qs].sort((a, b) =>
+    a.updatedAt < b.updatedAt ? 1 : a.updatedAt > b.updatedAt ? -1 : 0,
+  );
+}