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/lib/api/plex.ts

@@ -390,6 +390,8 @@ export interface PlexPlaySession {
   mode: "direct" | "hls";
   url: string;
   start_s: number;
+  transcode?: boolean; // hls only: video re-encoded (non-h264 source) vs stream-copied (remux)
+  hw?: boolean; // hls transcode only: ran on the VAAPI hardware encoder (else libx264 software)
 }
 
 // Discover surface (E5 S4): the consolidated rail payload. Every rail is a plain PlexCard list the