feat(plex): up-next end-card at episode end (E6 S4)

At episode end, when a next item exists and the pref is on, show an up-next
card (poster + title + ~10s countdown + Cancel) instead of the old instant jump,
so binge stays in the viewer's control (D6-A). Countdown or Play now advances;
Cancel/✕ dismisses and stays on the finished frame. Lives only on the
play-to-the-end path — credits-auto-skip advances earlier and never reaches the
card, so the two reconcile by construction (no double-advance). New pref
'upNext' (default on) with a Playback-tab toggle; the card closes on item change
and joins the Back/Escape cascade. i18n HU/EN.

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

@@ -192,6 +192,12 @@
       "fineSeek": "Fine seek",
       "stats": "Stats for nerds"
     },
+    "upNext": {
+      "title": "Up next",
+      "playNow": "Play now",
+      "cancel": "Cancel",
+      "pref": "Up-next card"
+    },
     "stats": {
       "title": "Stats for nerds",
       "mode": "Mode",

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

@@ -192,6 +192,12 @@
       "fineSeek": "Finom léptetés",
       "stats": "Statisztika (nerdöknek)"
     },
+    "upNext": {
+      "title": "Következik",
+      "playNow": "Lejátszás most",
+      "cancel": "Mégse",
+      "pref": "Következik-kártya"
+    },
     "stats": {
       "title": "Statisztika (nerdöknek)",
       "mode": "Mód",