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/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.",