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/VideoCard.tsx

@@ -1,4 +1,4 @@
-import { Bookmark, Check, Eye, EyeOff, ListFilter } from "lucide-react";
+import { Bookmark, Check, CheckCheck, Eye, EyeOff, ListFilter } from "lucide-react";
 import clsx from "clsx";
 import type { Video } from "../lib/api";
 import { formatDuration, formatViews, relativeTime } from "../lib/format";
@@ -21,20 +21,24 @@ function Actions({
     <div className="flex gap-1 opacity-0 group-hover:opacity-100 transition">
       <button
         onClick={act("watched")}
-        title="Mark watched"
+        title={video.status === "watched" ? "Watched — click to unmark" : "Mark watched"}
         className={clsx(
           "p-1.5 rounded-md hover:bg-surface text-muted hover:text-fg",
           video.status === "watched" && "text-accent"
         )}
       >
-        <Check className="w-4 h-4" />
+        {video.status === "watched" ? (
+          <CheckCheck className="w-4 h-4" />
+        ) : (
+          <Check className="w-4 h-4" />
+        )}
       </button>
       <button
         onClick={act("saved")}
-        title="Save for later"
+        title={video.status === "saved" ? "Saved — click to remove" : "Save for later"}
         className={clsx(
           "p-1.5 rounded-md hover:bg-surface text-muted hover:text-fg",
-          video.status === "saved" && "text-accent"
+          video.status === "saved" && "fill-current text-accent"
         )}
       >
         <Bookmark className="w-4 h-4" />