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>

frontend/src/pages/SearchResults.jsx

@@ -108,9 +108,9 @@ export default function SearchResults() {
               {hasMore ? `${visibleCount} of ${videos.length}` : videos.length}
             </span>
           </h2>
-          <div className="grid grid-cols-2 sm:grid-cols-3 lg:grid-cols-4 xl:grid-cols-5 gap-4">
+          <div className="flex flex-col gap-2">
             {visibleVideos.map((v) => (
-              <VideoCard key={v.youtube_video_id} video={v} />
+              <VideoCard key={v.youtube_video_id} video={v} variant="list" />
             ))}
           </div>
           {hasMore && (