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/components/Navbar.vue

@@ -7,6 +7,12 @@
       <RouterLink to="/search" class="nav-link" active-class="active">搜索</RouterLink>
       <RouterLink to="/library" class="nav-link" active-class="active">音乐库</RouterLink>
       <RouterLink to="/history" class="nav-link" active-class="active">播放历史</RouterLink>
+      <RouterLink
+        v-if="store.savedQueuesEnabled && !session.isGuest.value"
+        to="/saved-queues"
+        class="nav-link"
+        active-class="active"
+      >已存队列</RouterLink>
     </div>
 
     <div class="nav-right">