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/routes/feed.py

@@ -6,7 +6,7 @@ from sqlalchemy.orm import Session, aliased
 
 from app.auth import current_user
 from app.db import get_db
-from app.models import Channel, ChannelTag, Tag, User, Video, VideoState
+from app.models import Channel, ChannelTag, Subscription, Tag, User, Video, VideoState
 
 router = APIRouter(prefix="/api", tags=["feed"])
 
@@ -78,14 +78,26 @@ def get_feed(
             status_expr,
         )
         .join(Channel, Channel.id == Video.channel_id)
+        # Only channels this user is subscribed to (and hasn't hidden).
+        .join(
+            Subscription,
+            and_(
+                Subscription.channel_id == Video.channel_id,
+                Subscription.user_id == user.id,
+                Subscription.hidden.is_(False),
+            ),
+        )
         .outerjoin(
             state, and_(state.video_id == Video.id, state.user_id == user.id)
         )
     )
 
-    if not include_shorts:
+    # In the explicit Watched/Saved/Hidden views, show the complete set regardless of
+    # the Shorts / live default-hiding (so e.g. a hidden Short still shows up there).
+    explicit_view = show in ("watched", "saved", "hidden")
+    if not include_shorts and not explicit_view:
         query = query.where(Video.is_short.is_(False))
-    if not include_live:
+    if not include_live and not explicit_view:
         query = query.where(Video.live_status.notin_(HIDDEN_LIVE))
     if channel_id:
         query = query.where(Video.channel_id == channel_id)