feat(player): keyboard + scroll-wheel controls in the video modal

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.

frontend/src/i18n/locales/en/player.json

@@ -19,5 +19,6 @@
   "unavailableTitle": "Can't play this here",
   "unavailableBody": "This video isn't available — it may be private, removed, or region-restricted.",
   "embedDisabledBody": "The owner disabled playback on other sites (common for auto-generated “Topic” music tracks). It still plays on YouTube.",
-  "openOnYoutube": "Open on YouTube"
+  "openOnYoutube": "Open on YouTube",
+  "shortcutsHint": "Click: play/pause · Scroll: volume · F: fullscreen"
 }