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/lib/plexPlayerPrefs.ts

@@ -45,6 +45,7 @@ export type PlexPlayerPrefs = {
   autoSkipIntro: boolean; // auto-skip the intro marker after a countdown
   autoSkipCredits: boolean; // auto-skip credits → jump to the next item (binge)
   autoSkipDelay: number; // countdown before an auto-skip fires (s, 0 = immediate)
+  upNext: boolean; // at episode end (with a next item) show an up-next card + countdown before advancing
 };
 export const DEFAULT_PREFS: PlexPlayerPrefs = {
   volume: 1,
@@ -71,6 +72,7 @@ export const DEFAULT_PREFS: PlexPlayerPrefs = {
   autoSkipIntro: false,
   autoSkipCredits: false,
   autoSkipDelay: 5,
+  upNext: true,
 };
 export const clamp01 = (n: number) => Math.max(0, Math.min(1, n));