feat(player): step through the feed, reachable native controls, hover-intent

Three in-app player refinements:
- Prev/next stepping: the modal now takes the feed's loaded order as a queue, with
  faint arrow zones flanking the card and Shift+Left/Right shortcuts (plain arrows
  seek ±5s). The queue is frozen at open so marking the current video watched can't
  drop it and reload a different one; auto-advance-on-end stays off for the feed
  (it isn't a playlist) — playlists keep theirs.
- Reachable native YouTube controls: the interaction overlay (wheel volume / click
  pause / keyboard focus) now covers only the centre band (top-[12%] bottom-[22%]),
  leaving the top-right cluster (volume/CC/settings) and the bottom bar (seek /
  More videos / fullscreen) clickable. Verified live: settings menu + fullscreen work.
- Hover-intent description: the title popover now waits ~400ms so a quick pass no
  longer flashes it.

frontend/src/components/Feed.tsx

@@ -446,6 +446,8 @@ export default function Feed({
             <PlayerModal
               video={activeVideo.video}
               startAt={activeVideo.startAt}
+              queue={ytItems}
+              autoAdvance={false}
               onClose={() => setActiveVideo(null)}
               onState={onState}
               onOpenChannel={onOpenChannel}
@@ -654,6 +656,8 @@ export default function Feed({
           <PlayerModal
             video={activeVideo.video}
             startAt={activeVideo.startAt}
+            queue={items}
+            autoAdvance={false}
             onClose={() => setActiveVideo(null)}
             onState={onState}
             onOpenChannel={onOpenChannel}