fix(player): first-class maximise/fullscreen view (UAT round)

UAT feedback on the F-maximise view — three fixes:
- Own control bar in the big views (maximise + our fullscreen): previous/next +
  N/M, auto-advance, loop, add-to-playlist, download, mark-watched, exit, and a
  true-fullscreen toggle. Auto-hides on idle. Add/download are hidden in true
  fullscreen (their popovers portal to <body>, outside the fullscreen element).
- Wheel volume works in the big views again: the centre interaction overlay is
  now armed in maximise/fullscreen (it only yields to YouTube's own settings
  menu), so the wheel bubbles to our handler instead of the bare iframe. F also
  offers true fullscreen that WE own (Fullscreen API on the stage), so wheel/
  arrow volume and our controls work inside it too — YouTube's own fullscreen
  button, which we never surface, is not involved.
- Back (mouse/browser) steps out one level at a time via nested history layers
  (useBackToExit): fullscreen -> maximise -> the modal -> closed, instead of
  closing the whole player at once.

Verified in Chrome: wheel volume in maximise + fullscreen, the control bar, and
the full Back ladder (fullscreen -> maximise -> modal -> closed).

frontend/src/lib/history.ts

@@ -114,3 +114,26 @@ export function useBackToClose(onClose: () => void): void {
     };
   }, []);
 }
+
+/** A Back-closable layer that exists only WHILE `active`. Stacks on top of `useBackToClose` and any
+ * other active `useBackToExit` (LIFO) so nested views peel off one Back at a time — e.g. a player's
+ * fullscreen sits above its maximise, which sits above the player itself: Back exits fullscreen,
+ * then maximise, then closes the player. Leaving a layer by its own control (button/Esc) flips
+ * `active` false, whose cleanup drops the entry and its history state on the next tick — exactly the
+ * useBackToClose lifecycle, just gated on a flag. `onExit` runs on a genuine Back while active. */
+export function useBackToExit(active: boolean, onExit: () => void): void {
+  const cb = useRef(onExit);
+  cb.current = onExit;
+  useEffect(() => {
+    if (!active) return;
+    ensureOverlayListener();
+    const entry: Overlay = { token: ++overlayCounter, cb: () => cb.current() };
+    overlayStack.push(entry);
+    scheduleReconcile();
+    return () => {
+      const idx = overlayStack.indexOf(entry);
+      if (idx !== -1) overlayStack.splice(idx, 1);
+      scheduleReconcile();
+    };
+  }, [active]);
+}

frontend/src/lib/releaseNotes.ts

@@ -20,8 +20,8 @@ export const RELEASE_NOTES: ReleaseEntry[] = [
     summary:
       "A sturdier player — press F to go big, arrow-key volume that's remembered — plus downloads and subscription sync that hold up when something goes wrong.",
     features: [
-      "Player: press F to maximise the video to a full-window view, and use the ↑/↓ arrow keys to change the volume — your level is now remembered per account. Ctrl+scroll zooms the page as usual again instead of being captured by the player.",
-      "The maximised view no longer blocks YouTube's own fullscreen button or on-video controls, keyboard shortcuts keep working after you click into the video, and exiting the view returns keyboard focus to the app. Both the YouTube and the local/Plex player behave the same way.",
+      "Player: press F to maximise the video to a full-window view — now with its own control bar (previous/next, auto-advance, loop, add-to-playlist, download, mark watched) that auto-hides while you watch, plus a button for true fullscreen. Scroll-wheel and ↑/↓ volume work in every mode (the full window and fullscreen too), and your volume is remembered per account.",
+      "Volume: Ctrl+scroll zooms the page as usual again instead of being captured by the player. Keyboard shortcuts keep working after you click into the video, and the mouse/browser Back button now steps out one level at a time (fullscreen → full window → the normal player → closed) instead of closing the whole player at once.",
     ],
     fixes: [
       "Downloads: fixed a class of downloads that could fail with an internal error (a produced file the downloader couldn't locate), and mp4 downloads are no longer needlessly re-encoded — they finish faster and keep their original quality.",