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/Discovery.jsx

@@ -324,11 +324,12 @@ export default function DiscoveryPage() {
         </>
       ) : (
         <>
-          <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, is_recommended: true }}
+                variant="list"
                 onDismiss={handleDismissVideo}
               />
             ))}