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/App.vue

@@ -166,6 +166,9 @@ onMounted(async () => {
   // Search or Playlist render hearts correctly without first visiting Home.
   // (fire-and-forget; fetchFavorites swallows the 401 when not yet logged in.)
   playerStore.fetchFavorites();
+  // Non-critical: reads savedQueuesEnabled so the nav entry can show/hide.
+  // Guests get a 403 (swallowed) → the entry stays hidden for them.
+  if (!session.isGuest.value) playerStore.fetchBotSettings();
   syncTimer = setInterval(() => playerStore.syncElapsed(), 3000);
   mobileRaf = requestAnimationFrame(updateMobileProgress);
   // Reconcile the dedicated-link scope only after the bot list is known: the