Improve frontend navigation and mobile UX

- Add mobile bottom tab bar for media browsing (Home, Libraries, Favourites, Playlists, More)
- Split mobile drawer: media links first, admin links under a Management section
- Show contextual back button in mobile header for deep routes
- Add reusable PageBackButton and PageHeader components
- Add back navigation to playlist detail, library page, and missing media state
- Embed queue panels in TaskQueuePage to avoid duplicate page titles
- Expose theme toggle in user menu on small screens

Co-authored-by: truewhile <truewhile@users.noreply.github.com>

web/src/pages/MediaDetailPageSections.tsx

@@ -1,6 +1,8 @@
 import { ArrowLeft, Heart, Play, RefreshCw } from 'lucide-react'
 import { Link } from 'react-router-dom'
 
+import { PageBackButton } from '../components/PageBackButton'
+
 import { ExternalPlayerButton } from '../components/ExternalPlayerButton'
 import { ManualScrapeDialog } from '../components/ManualScrapeDialog'
 import { MetadataEditDialog } from '../components/MetadataEditDialog'
@@ -63,8 +65,11 @@ export function MediaDetailLoading() {
 
 export function MediaDetailMissing() {
   return (
-    <div className="text-center py-24 bg-white rounded-2xl border border-gray-200">
-      <p className="text-gray-500">媒体资源已被移除或不存在</p>
+    <div className="space-y-4 py-12 text-center">
+      <PageBackButton label="返回上一页" className="mx-auto" />
+      <div className="rounded-2xl border border-gray-200 bg-white py-16">
+        <p className="text-gray-500">媒体资源已被移除或不存在</p>
+      </div>
     </div>
   )
 }