fix(plex): player marker-skip reset, lang-switch reload, tracks wheel, sidebar badge clamp

- PP1: reset cancelledMarkerRef on item change — a cancelled intro/credits
  auto-skip on one episode suppressed a same-offset marker on the next (auto-skip
  silently dead for the rest of a binge).
- PP2: loadSession no longer depends on the i18n `t` (read via a ref). A
  mid-playback language switch rebuilt loadSession → re-ran the detail effect →
  reloaded the session from the STALE saved resume position, losing live progress.
- PP3: tracks (audio/subtitle) menu now stopPropagation on wheel, like the gear
  menu — scrolling the list no longer changes volume.
- PS1: collapsed PlexSidebar filter badge clamps to "9+" (was a raw count),
  matching the feed Sidebar.

frontend/src/components/PlexSidebar.tsx

@@ -133,7 +133,7 @@ export default function PlexSidebar({
           <SlidersHorizontal className="w-5 h-5" />
           {activeCount > 0 && (
             <span className="absolute -top-1 -right-1 min-w-[16px] h-4 px-1 rounded-full bg-accent text-accent-fg text-[10px] font-bold leading-none grid place-items-center ring-2 ring-bg">
-              {activeCount}
+              {activeCount > 9 ? "9+" : activeCount}
             </span>
           )}
         </button>