fix: feedback round 2 — language, subscriptions, feed scope, UX

- Language detection: classify one cleaned+concatenated blob (strip emoji,
  @mentions, #tags, numbers, punctuation); fixes caps/emoji-heavy channels
  (e.g. Nessaj -> Hungarian, no more bogus Chinese/Korean)
- Feed now joins the user's subscriptions, so unsubscribing on YouTube removes a
  channel from the feed; periodic subscription re-sync job picks up changes
- Watched/Saved/Hidden views ignore the Shorts/live default-hiding so the full
  set is visible (fixes hidden videos missing from the Hidden view)
- Persist feed filters + search across reloads (localStorage)
- 3D polish: cards lift with shadow on hover; chips/buttons get depth and a
  press effect; undo toast lasts longer

frontend/src/lib/toast.ts

@@ -20,7 +20,7 @@ export function toast(message: string, action?: ToastAction): number {
   const id = counter++;
   items = [...items, { id, message, action }];
   emit();
-  setTimeout(() => dismiss(id), 6000);
+  setTimeout(() => dismiss(id), 9000);
   return id;
 }