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/i18n/locales/en/plex.json

@@ -155,6 +155,7 @@
   "player": {
     "loading": "Loading…",
     "buffering": "Buffering…",
+    "preparing": "Preparing stream…",
     "back": "Back",
     "skipIntro": "Skip intro",
     "skipCredits": "Skip credits",

frontend/src/i18n/locales/hu/plex.json

@@ -155,6 +155,7 @@
   "player": {
     "loading": "Betöltés…",
     "buffering": "Pufferelés…",
+    "preparing": "Stream előkészítése…",
     "back": "Vissza",
     "skipIntro": "Intro átugrása",
     "skipCredits": "Stáblista átugrása",