Adds player-modal shortcuts and keeps keyboard focus on the modal (not the
cross-origin player iframe) on open and on player-ready, so they work until you
click into YouTube's native controls:
- F: toggle fullscreen (the player stage element, so the volume flash stays
visible; a :fullscreen CSS rule drops the 16:9 letterbox). Esc in fullscreen
exits fullscreen only, without also closing the modal.
- Space: play/pause (ignored while a button/input is focused).
- Scroll wheel over the video: volume up/down with an auto-fading volume-bar
overlay. A transparent interaction layer over the iframe catches the wheel
(the cross-origin iframe otherwise swallows it) and maps click to play/pause;
it leaves the bottom strip uncovered so YouTube's native control bar (seek /
settings / captions / fullscreen) stays usable, and is hidden on an embed
error so the 'Open on YouTube' CTA stays clickable.
i18n shortcutsHint EN/HU/DE.
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