fix(plex): player + card polish from UAT

- Player: control tooltips now open ABOVE the button (no downward viewport clip);
  keyboard use also reveals the auto-hidden controls (wake() on keydown); added a
  Download button that downloads the ORIGINAL physical file (no re-encode/repackage,
  Content-Disposition attachment via GET /stream/{rk}/file?download=1).
- Plex cards: hover affordance shows what a click does (Play / Resume / Open show)
  + a quick watched/unwatched toggle (stopPropagation, no playback) — mirrors the
  YT-feed card quick-actions. New plex i18n keys en/hu/de.
- Verified in a real browser: card hover Play overlay + watched toggle (marks watched
  without opening), player download button present, tooltip-above, keyboard reveals
  controls, correct durations (Enola Holmes 3 = 1:48:04).
- NOTE: 'The Three Diablos' showing 13:05 was NOT a bug — it is a 13-min short (file
  ffprobe = 785s); the 1h42m film is 'The Last Wish'.

frontend/src/i18n/locales/de/plex.json

@@ -25,6 +25,11 @@
   "empty": "Noch nichts hier. Starte eine Plex-Synchronisierung auf der Admin-Konfigurationsseite.",
   "loadMore": "Mehr laden",
   "watched": "Angesehen",
+  "play": "Abspielen",
+  "resume": "Fortsetzen",
+  "openShow": "Serie öffnen",
+  "markWatched": "Als gesehen markieren",
+  "markUnwatched": "Als ungesehen markieren",
   "seasons": "{{count}} Staffeln",
   "playerSoon": "Player kommt bald — „{{title}}“",
   "player": {
@@ -37,7 +42,8 @@
     "prev": "Vorherige Folge",
     "next": "Nächste Folge",
     "mute": "Stumm (m)",
-    "fullscreen": "Vollbild (f)"
+    "fullscreen": "Vollbild (f)",
+    "download": "Originaldatei herunterladen"
   },
   "playable": {
     "direct": "Spielt direkt im Browser",