UX: list view everywhere, mobile polish, affinity dismissal fix

- Default list view across all pages (Home, Following, History, Queue,
  ContinueWatching, Liked, Discovery, SearchResults, Channel)
- Watch.jsx mobile: smaller chips/title/avatar/meta, hide tags + keyboard
  hint on mobile, tighter gaps, compact description padding
- Fix mobile bottom nav showing focus outline on tap
- Fix _update_affinity to write negative entries (not just positive) so
  dislikes/dismissals on unseen content actually register
- Dismissing a discovery video now fires -3.0 affinity against its tags,
  matching the dislike weight

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/routers/videos.py

@@ -37,8 +37,7 @@ def _update_affinity(db: Session, user_id: int, video: Video, delta: float):
             existing.score = max(existing.score + delta, -20.0)
             existing.updated_at = datetime.utcnow()
         else:
-            if delta > 0:
-                db.add(UserTagAffinity(user_id=user_id, tag=tag, score=delta))
+            db.add(UserTagAffinity(user_id=user_id, tag=tag, score=delta))