revert(player): drop the HD-latch resolution trick, keep the windowed fullscreen button

The large-logical-iframe + CSS-scale-down trick to force a 1080p start did not
work behind the user's work VPN (YouTube caps quality by network there regardless
of viewport size), and the two-phase native switch could downgrade a latched
1080p to 480p on some streams. Not worth the complexity for an unreliable win —
revert to the original "opens at whatever resolution YouTube picks" behaviour;
the user sets 1080p manually via fullscreen + the gear as before.

Kept: the windowed maximise (fullscreen) button added to the player meta row.

frontend/src/lib/youtube.ts

@@ -15,11 +15,6 @@ export interface YTPlayer {
   getDuration(): number;
   getVideoData(): YTVideoData;
   getIframe(): HTMLIFrameElement;
-  // Current latched playback quality (e.g. "hd1080") — used to detect when to drop the HD-latch
-  // iframe from its scaled-up size to native sizing. `setPlaybackQuality` is deliberately NOT typed:
-  // YouTube retired it (a hard no-op) — see [[reference-yt-iframe-api-remote-control-map]].
-  getPlaybackQuality(): string;
-  setSize(width: number, height: number): void;
   getVolume(): number;
   isMuted(): boolean;
   setVolume(volume: number): void;