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/de/player.json

@@ -19,5 +19,6 @@
   "unavailableTitle": "Hier nicht abspielbar",
   "unavailableBody": "Dieses Video ist nicht verfügbar — möglicherweise privat, entfernt oder regional gesperrt.",
   "embedDisabledBody": "Der Eigentümer hat die Wiedergabe auf anderen Seiten deaktiviert (häufig bei automatisch generierten „Topic“-Musiktiteln). Auf YouTube ist es weiterhin abspielbar.",
-  "openOnYoutube": "Auf YouTube öffnen"
+  "openOnYoutube": "Auf YouTube öffnen",
+  "shortcutsHint": "Klick: Wiedergabe/Pause · Scrollen: Lautstärke · F: Vollbild"
 }