feat(plex): player HTPC polish — Stop button, keyboard help, time-remaining + clock, scroll-restore

- Explicit Stop button (stops playback + returns to the library).
- Keyboard cheat-sheet overlay toggled with 'H' (movie hides the episode-nav row).
- Backspace = stop & back to feed (HTPC-remote convention); closes the help first.
- Time-remaining readout beside the seek bar + a live wall clock in the top bar
  (both fade with the controls).
- PlexBrowse restores the grid scroll position when returning from the player, so
  Back lands on the same card instead of the top of the library.
- New plex.player i18n keys (stop/help/keys.*) in en/hu/de.

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

@@ -50,7 +50,19 @@
     "subOff": "Off",
     "errNotFound": "This file can't be played — the media isn't reachable on the server (the Plex media mount is missing or the path map is wrong). Ask the admin to check the Plex configuration.",
     "errTranscode": "This file's format needs transcoding, which isn't supported yet.",
-    "errGeneric": "Playback couldn't start. Please try again."
+    "errGeneric": "Playback couldn't start. Please try again.",
+    "stop": "Stop",
+    "help": "Keyboard shortcuts",
+    "keys": {
+      "playPause": "Play / pause",
+      "seek": "Seek 10 seconds",
+      "episode": "Previous / next episode",
+      "volume": "Volume up / down",
+      "mute": "Mute",
+      "fullscreen": "Fullscreen",
+      "back": "Stop & back to feed",
+      "help": "Show this help"
+    }
   },
   "playable": {
     "direct": "Plays directly in the browser",