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/Channel.jsx

@@ -164,9 +164,9 @@ export default function ChannelPage() {
           <div className="flex justify-end">
             <SortPicker value={videoSort} onChange={setVideoSort} options={VIDEO_SORTS} />
           </div>
-          <div className="grid grid-cols-2 sm:grid-cols-3 lg:grid-cols-4 gap-4">
+          <div className="flex flex-col gap-2">
           {sortVideos(videos, videoSort).map((v) => (
-            <VideoCard key={v.youtube_video_id} video={{ ...v, channel_name: channel.name }} />
+            <VideoCard key={v.youtube_video_id} video={{ ...v, channel_name: channel.name }} variant="list" />
           ))}
           </div>
         </>