feat(player): webview audio backend for platforms without a native one

Adds WebviewAudioBackend, used on non-Linux/non-Android targets (e.g.
Windows) where there is no libmpv/ExoPlayer. Instead of decoding, it
emits a WebviewAudioLoad event with the stream URL; a frontend <audio>
element (WebviewAudioAdapter + webviewAudio service) plays it and reports
state/position back through the existing player_report_* round-trip, so
the Rust PlayerController stays the single source of truth. Play/pause/
seek reach the element via the existing ControlCommand event.

All video already renders in the webview on every platform, so this
completes audio-only playback for Windows (video via WebView2, audio via
<audio>). Pure Rust + Tauri events, so it still cross-compiles from Linux.

Regenerates bindings.ts (adds webview_audio_load; also carries the
equalizer EQ bindings).

TRACES: UR-003, UR-004, UR-005 | DR-004

src-tauri/src/player/mod.rs

@@ -22,6 +22,11 @@ pub mod android;
 #[cfg(target_os = "linux")]
 pub mod mpv_backend;
 
+// Platforms with no native audio backend (e.g. Windows) render audio-only
+// playback through a webview <audio> element, mirroring how all video renders.
+#[cfg(not(any(target_os = "linux", target_os = "android")))]
+pub mod webview_audio_backend;
+
 // Re-export commonly used types
 pub use autoplay::{AutoplayDecision, AutoplaySettings};
 pub use backend::{NullBackend, PlayerBackend, PlayerError};
@@ -40,6 +45,9 @@ pub use android::ExoPlayerBackend;
 #[cfg(target_os = "linux")]
 pub use mpv_backend::MpvBackend;
 
+#[cfg(not(any(target_os = "linux", target_os = "android")))]
+pub use webview_audio_backend::WebviewAudioBackend;
+
 #[cfg(target_os = "android")]
 pub use android::{
     disable_remote_volume, enable_remote_volume, get_detected_codecs, set_media_command_handler,