feat(plex): stats-for-nerds debug overlay (E6 S2)

Add a toggleable stats overlay to the Plex player: active mode
(direct/remux/transcode), transport, resolution, codec, bitrate, HLS level,
dropped frames, buffer-ahead, session offset and position — sampled once a
second off the live videoRef + hlsRef (no data layer). Non-modal corner panel,
aria-live polite, dismissable; toggled with 'd', a gauge control-bar button, or
its X. Number-crunching lives in a pure, unit-tested plexStats module. Doubles
as the S1 transcode-verification surface. i18n HU/EN.

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

@@ -189,7 +189,21 @@
       "help": "Show this help",
       "audio": "Cycle audio track",
       "subtitle": "Cycle subtitle",
-      "fineSeek": "Fine seek"
+      "fineSeek": "Fine seek",
+      "stats": "Stats for nerds"
+    },
+    "stats": {
+      "title": "Stats for nerds",
+      "mode": "Mode",
+      "transport": "Transport",
+      "resolution": "Resolution",
+      "codec": "Codec",
+      "bitrate": "Bitrate",
+      "level": "HLS level",
+      "frames": "Dropped frames",
+      "buffer": "Buffer ahead",
+      "offset": "Session offset",
+      "position": "Position"
     },
     "settings": "Settings",
     "sync": "Offsets",

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

@@ -189,7 +189,21 @@
       "help": "Súgó megjelenítése",
       "audio": "Hangsáv váltása",
       "subtitle": "Felirat váltása",
-      "fineSeek": "Finom léptetés"
+      "fineSeek": "Finom léptetés",
+      "stats": "Statisztika (nerdöknek)"
+    },
+    "stats": {
+      "title": "Statisztika (nerdöknek)",
+      "mode": "Mód",
+      "transport": "Átvitel",
+      "resolution": "Felbontás",
+      "codec": "Kodek",
+      "bitrate": "Bitráta",
+      "level": "HLS szint",
+      "frames": "Eldobott képkockák",
+      "buffer": "Puffer előre",
+      "offset": "Munkamenet-eltolás",
+      "position": "Pozíció"
     },
     "settings": "Beállítások",
     "sync": "Eltolások",