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/components/PlexPlayer.tsx

@@ -206,6 +206,10 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
       setReady(false);
       setBuffering(false);
       setLoadError(null);
+      // Clear the prior item's transcode flag up front so the cold-start overlay can't show a stale
+      // "Preparing stream…" for a not-yet-known session while the plexSession request is in flight —
+      // it's re-set honestly from the response below.
+      setSessionIsTranscode(false);
       let sess;
       try {
         // Multi-audio → request the multi-rendition session (no single `audio` ordinal); the track is
@@ -832,9 +836,13 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
 
       {/* While a panel/menu is open or an auto-skip countdown is running, take over the browser/mouse
           Back button (history popstate) so it closes/cancels that instead of dropping to the grid. */}
-      {(menuOpen || tracksOpen || helpOpen || infoOpen || skipProgress != null) && (
-        <BackClose onBack={backIntent} />
-      )}
+      {(menuOpen ||
+        tracksOpen ||
+        helpOpen ||
+        infoOpen ||
+        statsOpen ||
+        upNextOpen ||
+        skipProgress != null) && <BackClose onBack={backIntent} />}
 
       {/* Top bar */}
       <div
@@ -1235,7 +1243,7 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
               <button
                 onClick={() => setHelpOpen(false)}
                 className="p-1 rounded-lg hover:bg-white/10"
-                aria-label={t("plex.player.back")}
+                aria-label={t("plex.player.close")}
               >
                 <X className="w-5 h-5" />
               </button>