feat(player): watched controls, compact layout, description popover

- Watched: an explicit toggle in the modal (Mark watched / Watched→unmark) plus
  auto-mark when playback reaches the end (within 10s, or on the ended event).
- Compact layout: drop the header bar and the redundant 'Open on YouTube' button
  (the embed's own YouTube logo already jumps out); Close moves to the title row,
  channel + meta share one line — fits without a scrollbar at higher zoom.
- Card actions reflect status: watched shows a double-check, saved a filled
  bookmark, with matching tooltips.
- Description: new GET /api/videos/{id} exposes the already-stored description,
  shown in a popover when hovering the modal title (fetched lazily).

frontend/src/components/Feed.tsx

@@ -139,7 +139,11 @@ export default function Feed({
         </div>
       )}
       {activeVideo && (
-        <PlayerModal video={activeVideo} onClose={() => setActiveVideo(null)} />
+        <PlayerModal
+          video={activeVideo}
+          onClose={() => setActiveVideo(null)}
+          onState={onState}
+        />
       )}
       <div ref={sentinel} className="h-10" />
       {isFetchingNextPage && (