fix: the keyboard shortcuts the player has always advertised

The transport buttons have said "Play (space)" and "Full screen (f)"
since they were built, with nothing listening for either key. Pressing
f did nothing, which is exactly what a broken fullscreen button looks
like.

Both now work, verified in the running app: f enters and leaves
fullscreen, space plays and pauses. Space is prevented from also
pressing whichever button has focus, and neither fires while typing in
a field or alongside a modifier.

Escape now asks to leave fullscreen before the player considers
closing, so one keypress cannot drop you all the way back to the feed.

The button itself was never broken: it was checked on a downloaded
video, on a stream, and inside a window already in macOS fullscreen,
entering and leaving each time.

src/components/Player.tsx

@@ -299,8 +299,9 @@ export default function Player({
   // Escape backs out, as it does everywhere else in the app.
   useEffect(() => {
     const onKey = (e: KeyboardEvent) => {
-      // In fullscreen the browser already handles Escape; closing the player
-      // as well would drop you all the way back to the feed.
+      // In fullscreen, Escape is the transport bar's to handle: it leaves
+      // fullscreen. Closing the player as well would drop you all the way back
+      // to the feed in one keypress.
       if (e.key === "Escape" && !document.fullscreenElement) leave();
       // Arrow keys only when the video does not own them for seeking.
       if (e.key === "ArrowLeft" && e.shiftKey) onPrev?.();