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/utils/videoLinks.ts

@@ -0,0 +1,40 @@
+import type { FeedVideoDto } from '../api/client'
+
+interface LocationLike {
+  pathname: string
+  search: string
+}
+
+/**
+ * Where a video card links to: Shorts open the vertical feed at
+ * `/shorts/:id` (carrying the current list context so the feed keeps the
+ * same filter), everything else keeps the existing `/video/:id` page.
+ */
+export function videoHref(video: FeedVideoDto, location: LocationLike): string {
+  if (!video.is_short) return `/video/${video.youtube_video_id}`
+
+  const params = new URLSearchParams()
+  const { pathname, search } = location
+  const searchParams = new URLSearchParams(search)
+
+  if (pathname.startsWith('/category/')) {
+    const categoryId = pathname.split('/')[2]
+    if (categoryId) params.set('category', categoryId)
+  } else if (pathname === '/uncategorized') {
+    params.set('uncategorized', 'true')
+  } else if (pathname === '/local') {
+    params.set('downloaded', 'true')
+    const categoryId = searchParams.get('category')
+    if (categoryId) params.set('category', categoryId)
+    if (searchParams.get('uncategorized') === 'true') params.set('uncategorized', 'true')
+  } else if (pathname === '/search') {
+    const query = searchParams.get('q')
+    if (query) params.set('q', query)
+  } else if (pathname.startsWith('/channels/') && pathname.endsWith('/videos')) {
+    const channelId = pathname.split('/')[2]
+    if (channelId) params.set('channel', channelId)
+  }
+
+  const query = params.toString()
+  return `/shorts/${video.youtube_video_id}${query ? `?${query}` : ''}`
+}

frontend/src/utils/youtube.ts

@@ -0,0 +1,52 @@
+export interface YoutubePlayerApi {
+  seekTo(seconds: number, allowSeekAhead: boolean): void
+  getCurrentTime(): number
+  getDuration(): number
+  mute?(): void
+  unMute?(): void
+  isMuted?(): boolean
+  getVolume?(): number
+  destroy(): void
+}
+
+export interface YoutubePlayerCtorOptions {
+  videoId: string
+  host?: string
+  playerVars?: Record<string, string | number>
+  events?: {
+    onReady?: (event: { target: YoutubePlayerApi }) => void
+  }
+}
+
+declare global {
+  interface Window {
+    YT?: {
+      Player: new (element: HTMLElement, options: YoutubePlayerCtorOptions) => YoutubePlayerApi
+    }
+    onYouTubeIframeAPIReady?: () => void
+  }
+}
+
+let youTubeApiLoadPromise: Promise<void> | null = null
+
+export function loadYouTubeIframeApi(): Promise<void> {
+  if (window.YT?.Player) return Promise.resolve()
+  if (youTubeApiLoadPromise) return youTubeApiLoadPromise
+  youTubeApiLoadPromise = new Promise<void>((resolve, reject) => {
+    const previous = window.onYouTubeIframeAPIReady
+    window.onYouTubeIframeAPIReady = () => {
+      previous?.()
+      resolve()
+    }
+    const script = document.createElement('script')
+    script.src = 'https://www.youtube.com/iframe_api'
+    script.async = true
+    script.onerror = () => reject(new Error('YouTube IFrame API script failed to load'))
+    document.head.appendChild(script)
+  })
+  youTubeApiLoadPromise = youTubeApiLoadPromise.catch((error: unknown) => {
+    youTubeApiLoadPromise = null
+    throw error
+  })
+  return youTubeApiLoadPromise
+}