fix(plex): deep-review follow-ups on E6 transcode + player polish

Eight verified findings from the sprint deep-review:

- Transcode cap: _enforce_cap now counts only transcode sessions, so a
  live remux (stream-copy) stream can no longer be LRU-evicted mid-play —
  matching the config comment + the HU/EN Config hint that promise remux
  is uncapped.
- Player Back: the browser/mouse Back interceptor (BackClose) now also
  mounts for the stats overlay and up-next card, so Back closes them first
  instead of ejecting the whole player (keyboard/Esc already did).
- Honest cold-start: loadSession clears sessionIsTranscode up front, so a
  transcode->direct/remux switch no longer flashes a stale "Preparing
  stream..." during the session request.
- Stats overlay a11y: the close (X) buttons in the stats, help and info
  overlays now read as "Close"/"Bezaras" (new plex.player.close key), not
  "Back"; dropped aria-live from the per-second stat grid so a screen
  reader is not flooded with telemetry.

Tests: route-level transcode routing (transcode item -> HLS re-encode
session, remux stays copy, direct opens none); start_session threading the
preset/crf/max_height knobs + hw/preexec into the ffmpeg argv; UpNextCard
countdown auto-advance, Play-now, Cancel and the S?-E? heading fallback.

Also brings UpNextCard.tsx / plexStats.ts / plexStats.test.ts to prettier.

frontend/src/lib/plexStats.test.ts

@@ -119,7 +119,9 @@ describe("buildStatRows", () => {
   });
 
   it("shows an empty classification honestly as a dash", () => {
-    expect(buildStatRows({ ...base, playable: "" }).find((r) => r.key === "mode")?.value).toBe(DASH);
+    expect(buildStatRows({ ...base, playable: "" }).find((r) => r.key === "mode")?.value).toBe(
+      DASH,
+    );
   });
 });