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

backend/app/config.py

@@ -55,6 +55,7 @@ class Settings(BaseSettings):
     # Number of recent video titles sampled per channel for language detection.
     autotag_title_sample: int = 40
     autotag_interval_minutes: int = 30
+    subscriptions_resync_minutes: int = 360
     # live_status values hidden from the feed by default. Completed-stream VODs
     # ("was_live") are real watchable content and stay visible.
     feed_default_hidden_live: str = "live,upcoming"