refactor(player): delete the webview video path; mpv selects its own tracks

DR-235 phase 3. Every video renderer is native now: mpv on Linux and
Windows, ExoPlayer on Android, all drawing behind the transparent
webview. The HTML5 <video> path is gone, not bypassed:

- Frontend: hls.js, Html5PlayerAdapter and its compatibility shim, the
  createAdapter factory, streamTransport, hlsRecovery, timeTracking,
  videoFit, the <video>/<track> markup and every element handler in
  VideoPlayer (3277 -> 2144 lines), the experimentalNativeVideo store
  and its Settings toggle, webviewVideoFallback/supportsNativeVideo, and
  the setHtml5VideoState PiP bridge call. NativePlayerAdapter is the one
  video adapter; webview audio gets its own adapter kind.
- Rust: use_html5 dropped from player_seek_video,
  player_switch_audio_track and player_set_stream_quality with the
  Html5* strategies and ReloadStream responses; use_html5_element and
  VideoBackend dropped from PlayerStatus; player_play_item always loads
  the backend (set_current_item removed); Capabilities::webview removed;
  the WebKitGTK GStreamer/VAAPI setup (and its gst-inspect spawn) removed.
- Android: the HTML5 video state in PictureInPictureManager and
  ScreenWakeManager, and the bridge method feeding it.
- CSP: connect-src loses http:/https: and worker-src loses blob: -
  both existed for hls.js; with it gone they were only an exfiltration
  channel and a blob worker for injected script. A test now keeps them
  out.

mpv takes over what the <video> element did (mpv_tracks, UT-275):
subtitles are the WebVTT list the play request carries, queued on
sub-files and selected by position in that list, starting off; audio
tracks are selected by position in the file; sid/aid are reset before
each load. Without this, Linux video had no subtitle selection and a
direct-play audio switch failed since mpv became its renderer.

Verified: Rust 948 passing, and the same 948 cross-compiled for Windows
under wine against the shipped DLL (track tests included); frontend
1111 passing; aarch64 debug APK builds. Lint warnings 158 -> 146, CI
ratchet tightened to match. Not yet seen on Windows hardware.

src-tauri/src/player/mpv_backend.rs

@@ -384,8 +384,8 @@ impl MpvBackend {
             // StateChanged rather than tracking playback itself, per the
             // one-directional state rule. Unobserved, the event never came and
             // the button never moved. Invisible until native video shipped,
-            // because the webview <video> element's own DOM events drove that
-            // control on Linux.
+            // because the (since deleted) webview <video> element's own DOM
+            // events drove that control on Linux.
             //
             // TRACES: UR-005 | DR-239
             ev_ctx
@@ -696,6 +696,15 @@ impl PlayerBackend for MpvBackend {
         // TRACES: UR-040, UR-005 | DR-253
         *self.pending_seek.lock_safe() = None;
 
+        // The item's own sideloaded subtitles, none shown, and its default audio
+        // track — whatever the previous item had chosen. Only video carries
+        // subtitles; for audio this just clears the last item's.
+        // TRACES: UR-020, UR-021 | DR-023, DR-024, DR-235
+        let subtitle_urls: Vec<&str> = media.subtitles.iter().map(|t| t.url.as_str()).collect();
+        super::mpv_tracks::prepare_load(&self.mpv, &subtitle_urls).map_err(|e| PlayerError {
+            message: format!("Failed to prepare tracks: {e}"),
+        })?;
+
         // Load the media file. Through `mpv_command::command`, never
         // `Mpv::command`: the URL carries server-controlled text.
         // TRACES: UR-003, UR-004 | DR-298
@@ -858,6 +867,36 @@ impl PlayerBackend for MpvBackend {
         state.volume
     }
 
+    /// `stream_index` is a *position*: the n-th audio track of the file, the
+    /// same meaning ExoPlayer gives it (`player_switch_audio_track` passes the
+    /// array index). Only reached for a direct play/stream — a transcode carries
+    /// one track and is re-opened instead.
+    ///
+    /// TRACES: UR-021 | IR-019, DR-024, DR-235
+    fn set_audio_track(&mut self, stream_index: i32) -> Result<(), PlayerError> {
+        let position = usize::try_from(stream_index).map_err(|_| PlayerError {
+            message: format!("Invalid audio track position {stream_index}"),
+        })?;
+        super::mpv_tracks::select_audio(&self.mpv, position)
+            .map_err(|message| PlayerError { message })
+    }
+
+    /// `stream_index` is the position in the sideloaded subtitle list the play
+    /// request carried (`nativeSubtitleArrayIndex`), `None` to hide subtitles.
+    ///
+    /// TRACES: UR-020 | IR-018, DR-023, DR-235
+    fn set_subtitle_track(&mut self, stream_index: Option<i32>) -> Result<(), PlayerError> {
+        let position = stream_index
+            .map(|i| {
+                usize::try_from(i).map_err(|_| PlayerError {
+                    message: format!("Invalid subtitle position {i}"),
+                })
+            })
+            .transpose()?;
+        super::mpv_tracks::select_subtitle(&self.mpv, position)
+            .map_err(|message| PlayerError { message })
+    }
+
     fn set_audio_settings(&mut self, settings: &AudioSettings) -> Result<(), PlayerError> {
         info!("[MpvBackend] Applying audio settings");
         self.audio_settings = settings.clone();