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/router/index.ts

@@ -22,6 +22,7 @@ const router = createRouter({
     },
     { path: '/lyrics', name: 'lyrics', component: () => import('../views/Lyrics.vue') },
     { path: '/history', name: 'history', component: () => import('../views/History.vue') },
+    { path: '/saved-queues', name: 'saved-queues', component: () => import('../views/SavedQueues.vue') },
     { path: '/settings', name: 'settings', component: () => import('../views/Settings.vue') },
     { path: '/setup', name: 'setup', component: () => import('../views/Setup.vue') },
     { path: '/bot/:id', name: 'bot', component: () => import('../views/BotRedirect.vue') },