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/discovery.py

@@ -207,6 +207,10 @@ def dismiss_discovery_video(
     dq = db.query(DiscoveryQueue).filter_by(user_id=current_user.id, channel_id=channel_id).first()
     if dq:
         dq.seen = True
+
+    from ..routers.videos import _update_affinity
+    _update_affinity(db, current_user.id, video, -3.0)
+
     db.commit()