fix(desktop): load provider external subtitles into native player

[fmustafayaman]

Summary

On macOS/Windows the desktop native player surface received the externalSubtitles list in PlatformPlayerSurface but never forwarded it to NativePlayerSurface, so subtitles supplied by stream providers (getStreams → externalSubtitles) never reached mpv. As a result the in-player subtitle menu showed nothing under Built-in, even though the provider returned valid subtitle tracks.

This change:

• Forwards externalSubtitles from PlatformPlayerSurface into NativePlayerSurface.
• After the native player has attached the media, adds each subtitle URL via the existing setSubtitleUri → NativePlayerBridge.addSubtitleUrl (mpv sub-add ... select) path.

The Android engine (PlayerEngine.android.kt) already wires externalSubtitles into the player via MediaItem.SubtitleConfiguration; this brings the desktop native engine to parity.

Implementation notes

• Pure Kotlin change in composeApp/src/desktopMain/.../player/PlayerEngine.desktop.kt; no native (.mm / .cpp) changes required since addSubtitleUrl already exists in the bridge.
• Subtitles are added in a LaunchedEffect keyed on controller, sourceUrl, externalSubtitles and gated on hostFirstFullSizePaintComplete, with a short delay so the player has loaded the media before sub-add is issued.

Test plan

• Built locally on macOS arm64 (./gradlew :composeApp:run).
• Played a stream from a provider that returns externalSubtitles; logs show setSubtitleUri being invoked with a valid player handle and the subtitle track appearing/selectable under Built-in (previously empty).
• Verify on Windows native player.

Made with Cursor