feat(plex): honest transcode cold-start + encoder in stats (E6 S1d)

Surface the transcode session's runtime facts in the player. The session
response's transcode/hw flags now thread through: the stats overlay's mode row
reads "transcode · GPU" (VAAPI) or "transcode · CPU" (libx264) — the at-a-glance
confirmation of which S1 path engaged — and the initial-load overlay says
"Preparing stream…" instead of "Loading…" for a transcode session, since its
first segment is genuinely slower than a remux and shouldn't read as a hang.

fmtMode (pure, unit-tested) appends the encoder only for a transcode with a known
encoder. i18n HU/EN for the new preparing label. Browser-UAT'd on the XviD
fixture: mode row reads "transcode · CPU" live (software, no render node here).

frontend/src/components/PlexStatsOverlay.tsx

@@ -12,6 +12,7 @@ export function PlexStatsOverlay({
   videoRef,
   hlsRef,
   playable,
+  hwRef,
   sessionStartRef,
   absRef,
   durationRef,
@@ -20,6 +21,7 @@ export function PlexStatsOverlay({
   videoRef: RefObject<HTMLVideoElement | null>;
   hlsRef: RefObject<Hls | null>;
   playable: string;
+  hwRef: RefObject<boolean | null>; // active transcode encoder: true=GPU, false=CPU, null=not transcode
   sessionStartRef: RefObject<number>;
   absRef: RefObject<number>;
   durationRef: RefObject<number>;
@@ -75,6 +77,7 @@ export function PlexStatsOverlay({
       setRows(
         buildStatRows({
           playable,
+          hwEncoder: hwRef.current,
           transport: hls ? "hls" : "direct",
           videoWidth: v?.videoWidth ?? 0,
           videoHeight: v?.videoHeight ?? 0,
@@ -91,7 +94,7 @@ export function PlexStatsOverlay({
     sample();
     const iv = setInterval(sample, 1000);
     return () => clearInterval(iv);
-  }, [videoRef, hlsRef, playable, sessionStartRef, absRef, durationRef]);
+  }, [videoRef, hlsRef, playable, hwRef, sessionStartRef, absRef, durationRef]);
 
   return (
     <div