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.ts

@@ -109,7 +109,11 @@ export function buildStatRows(s: PlexStatsInput): StatRow[] {
   const rows: StatRow[] = [
     { key: "mode", labelKey: "plex.player.stats.mode", value: fmtMode(s.playable, s.hwEncoder) },
     { key: "transport", labelKey: "plex.player.stats.transport", value: s.transport },
-    { key: "res", labelKey: "plex.player.stats.resolution", value: fmtRes(s.videoWidth, s.videoHeight) },
+    {
+      key: "res",
+      labelKey: "plex.player.stats.resolution",
+      value: fmtRes(s.videoWidth, s.videoHeight),
+    },
   ];
   if (s.transport === "hls") {
     rows.push(
@@ -118,7 +122,11 @@ export function buildStatRows(s: PlexStatsInput): StatRow[] {
         labelKey: "plex.player.stats.codec",
         value: fmtCodec(s.hlsLevel?.videoCodec, s.hlsLevel?.audioCodec),
       },
-      { key: "bitrate", labelKey: "plex.player.stats.bitrate", value: fmtBitrate(s.hlsLevel?.bitrate) },
+      {
+        key: "bitrate",
+        labelKey: "plex.player.stats.bitrate",
+        value: fmtBitrate(s.hlsLevel?.bitrate),
+      },
       { key: "level", labelKey: "plex.player.stats.level", value: fmtLevel(s.hlsLevel) },
     );
   }