Split shorts from regular videos with a vertical shorts feed

- Detect shorts by duration (SHORTS_MAX_DURATION_SECONDS, default 180)
  and expose is_short in feed, video and channel-activity DTOs.
- Feed gains type=all|long|short and anchor; lists show a two-mode
  'Обычные | Shorts' filter (no 'Все') persisted in the URL.
- Clicking a short opens /shorts/🆔 a vertical scroll-snap feed with
  autoplay for the active slide, context-aware endpoints and infinite
  loading.
- Keep the sound choice across swipes, syncing with the player's own
  mute control and guarding against the widget's stale isMuted() reads.

frontend/src/App.tsx

@@ -7,6 +7,7 @@ import Feed from './pages/Feed'
 import Channels from './pages/Channels'
 import ChannelVideos from './pages/ChannelVideos'
 import VideoPage from './pages/VideoPage'
+import ShortsFeed from './pages/ShortsFeed'
 import Categories from './pages/Categories'
 import Settings from './pages/Settings'
 import './App.css'
@@ -29,6 +30,7 @@ function App() {
       <Route path="/channels" element={<Channels />} />
       <Route path="/channels/:channelId/videos" element={<ChannelVideos />} />
       <Route path="/video/:youtubeVideoId" element={<VideoPage />} />
+      <Route path="/shorts/:youtubeVideoId" element={<ShortsFeed />} />
       <Route path="/settings" element={<Settings email={data.email} />} />
       <Route path="/settings/categories" element={<Categories />} />
       <Route path="/categories" element={<Navigate to="/settings/categories" replace />} />