feat(downloads): guarantee every download is browser-playable (mp4/H.264/AAC)

The Download module's purpose is download -> share -> play in-browser, so the
output must play in EVERY browser (iOS/Safari/WebKit included). The only
universally-safe combo is mp4 + H.264 + AAC/MP3.

- formats.normalize() now CLAMPS every spec to that profile (container->mp4,
  vcodec->None, audio_format opus->m4a) at the single point all paths flow
  through (picker, stored profile, raw API) — not just by hiding UI options. A
  source exposing no H.264 is still caught by the worker's browser-safe
  re-encode, so playability is guaranteed end-to-end.
- build_ydl_opts drops the now-unreachable non-compat branches (+ _VCODEC_SORT).
- _SIG_VERSION 3->4 so assets cached under an old non-compat spec — or a stale
  non-H.264 mp4 that slipped the v3 net (the HEVC Facebook dls) — re-derive.
- ProfileEditor drops the container/codec pickers and Opus; audio is M4A/MP3.
- i18n: remove the now-unused container/vcodec/any keys (HU+EN parity).

frontend/src/i18n/locales/en/downloads.json

@@ -151,10 +151,7 @@
     "modeA": "Audio only",
     "quality": "Max quality",
     "best": "Best",
-    "container": "Container",
     "audioFormat": "Audio format",
-    "vcodec": "Video codec",
-    "any": "Any",
     "embedSubs": "Embed subtitles",
     "embedChapters": "Embed chapters",
     "embedThumbnail": "Embed thumbnail",

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

@@ -151,10 +151,7 @@
     "modeA": "Csak hang",
     "quality": "Max. minőség",
     "best": "Legjobb",
-    "container": "Konténer",
     "audioFormat": "Hangformátum",
-    "vcodec": "Videó codec",
-    "any": "Bármelyik",
     "embedSubs": "Feliratok beágyazása",
     "embedChapters": "Fejezetek beágyazása",
     "embedThumbnail": "Bélyegkép beágyazása",