fix(player): render subtitle tracks on the Linux HTML5 path (UR-020)

Selecting a subtitle on Linux did nothing. VideoPlayer rendered no <track>
children at all — the block was commented out as "temporarily disabled to
debug playback issues" (it has been that way since the POC) — so
Html5PlayerAdapter.selectSubtitle() walked an empty textTracks list and the
menu, which is built from media.mediaStreams, was purely decorative.

The reason it had to be disabled is still visible in the dead markup:
getSubtitleUrl() is async, so src={getSubtitleUrl(track.index)} bound a
Promise to the attribute and every track pointed at "[object Promise]" — an
unloadable resource hanging off the media element.

Subtitle URLs are now resolved off the render path into component state
(subtitleTracks.ts), and only streams whose URL actually resolved are
rendered; a per-track failure drops that track instead of emitting a dead
src. data-stream-index is kept, since that is what the adapter matches on.

Subtitles stay OFF unless the user asks for them: the server's isDefault flag
is shown in the menu but is never promoted to a selection, and the `default`
attribute is deliberately not emitted. A <track default> auto-shows, so the
menu would open on "Off" while subtitles were burned over the picture, and
every user who never wanted subtitles would suddenly get them. That matches
the existing initial state (selectedSubtitleIndex = null).

Selection and rendered tracks are reconciled whenever the list changes: a
selection that no longer resolves collapses to "Off", and a surviving one is
re-applied after the new <track> elements exist. "Off" disables every text
track, as before.

Cross-origin text-track fetches use the media element's CORS setting, so the
element opts in with crossorigin="anonymous" — but only for an http(s)
stream, never for a local/offline file:/asset: source, where forcing CORS
onto the video fetch could break playback. It is keyed on the subtitle stream
count, known at first render, so the attribute cannot flip under an in-flight
media load.

Android/native is untouched: the ExoPlayer branch still goes through
player_set_subtitle_track.

Tests (UT-143, UT-144) were written first and failed against the old markup:
the commented-out block, the Promise bound to src, and the default attribute.

TRACES: UR-020 | DR-023 | UT-143, UT-144

docs/requirements.md

@@ -542,6 +542,8 @@ Internal architecture, components, and application logic.
 | UT-140 | `useOfflineFilterReload` skips the value a page already loaded under and reloads on each later change | DR-143 | Done |
 | UT-141 | The advertised channel cap: an unknown or zero reading falls back to stereo, a real route keeps its channels, an absurd driver reading is capped at 7.1, and mono is taken at its word | DR-141 | Done |
 | UT-142 | The audio codecs offered for video direct play: a Dolby device's real `MediaCodecList` output drops `ac3`/`eac3`, AMR and raw PCM are dropped too, a fully-supported list is passed through untouched, a list with nothing decodable still claims `aac`, and stray spacing or casing does not decide whether the user gets sound | DR-148 | Done |
+| UT-143 | Subtitle URLs resolve to plain strings before they reach the markup (never a Promise), unresolvable tracks are dropped, a stale selection collapses to "Off", and a server-default track is never auto-selected | UR-020, DR-023 | Done |
+| UT-144 | VideoPlayer actually renders `<track kind="subtitles">` children carrying `data-stream-index`, with no `default` attribute and no async `getSubtitleUrl()` bound to `src` | UR-020, DR-023 | Done |
 
 ### Integration Tests