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/components/PlexPlayerSettings.tsx

@@ -113,6 +113,11 @@ export function PlexPlayerSettings(props: {
             checked={prefs.autoSkipCredits}
             onChange={(v) => patchPrefs({ autoSkipCredits: v })}
           />
+          <Toggle
+            label={t("plex.player.upNext.pref")}
+            checked={prefs.upNext}
+            onChange={(v) => patchPrefs({ upNext: v })}
+          />
           {(prefs.autoSkipIntro || prefs.autoSkipCredits) && (
             <Slider
               label={t("plex.player.autoSkipDelay")}