fix(player): change the audio track, and load subtitles at all

Two faults, both present since v0.0.1, both found and confirmed on a device.

Audio track (DR-258). Jellyfin builds a transcode around one AudioStreamIndex,
so the alternate tracks are not in the stream that arrives — but the native
path only ever called setAudioTrack(n), which indexes ExoPlayer's audio track
*groups*. On Android that is the common case, since any source whose default
audio codec the device cannot decode is transcoded: logcat showed ExoPlayer
holding `Audio tracks: 1` while the menu listed every track in the file, so
each selection warned `Invalid audio track index` and was dropped, leaving the
default track playing with nothing in the UI saying so.

determine_audio_track_switch_strategy now decides by whether the stream in
front of the engine carries the track at all — a direct play still selects in
place, a transcode is re-negotiated at the chosen index and resumed. Where it
resumes is the player's answer rather than the UI's: the native path has no
<video> element to read, so it sends no position, and defaulting that to zero
re-opened the film at the beginning (caught on device before it shipped).

Subtitles (DR-259). The URL was missing its `Stream.` route segment, so every
sideloaded subtitle 404ed; since media3 1.5 a sideloaded text track only
becomes a track group once its file is parsed, so 42 failed fetches left
ExoPlayer with no text tracks and selection warned `available: 0`. Verified
against a live server: the built URL answers 404, the corrected one 200. The
tests that should have caught this asserted the shape of a mock helper that
restated the format string instead of the URL the app requests — so the new
test drives the repository itself, and failed red on the old URL.

src-tauri/src/player/mod.rs

@@ -23,6 +23,7 @@ pub mod session;
 pub mod sleep_timer;
 pub mod state;
 pub mod stream_end;
+pub mod track_switch;
 
 #[cfg(test)]
 mod mpv_backend_test;
@@ -71,6 +72,7 @@ pub use seek::{determine_video_seek_strategy, VideoSeekStrategy};
 pub use session::{MediaSessionManager, MediaSessionType};
 pub use sleep_timer::{SleepTimerMode, SleepTimerState};
 pub use state::{EndReason, PlayerState};
+pub use track_switch::{determine_audio_track_switch_strategy, AudioTrackSwitchStrategy};
 
 // Re-export platform-specific backends
 #[cfg(target_os = "android")]