Linux: render video via HTML5 WebView instead of the MPV backend

- online.rs: on Linux, advertise only WebView-decodable codecs (h264 video;
  aac/mp3/opus/vorbis/flac audio) in PlaybackInfo so Jellyfin transcodes
  HEVC/AV1/VP9/etc. to h264 HLS for the WebKitGTK <video> element.
- player: on Linux, don't load video into MPV (no embedded window — it would
  start a redundant decode the frontend immediately stops). Add
  PlayerController::set_current_item to keep queue/UI/remote-transfer state in
  sync without loading the item into the playback backend.

src-tauri/src/repository/online.rs

@@ -859,7 +859,18 @@ impl MediaRepository for OnlineRepository {
                 ("h264,hevc".to_string(), "aac,mp3".to_string())
             });
 
-        #[cfg(not(target_os = "android"))]
+        // Linux desktop plays video through the WebKitGTK HTML5 <video> element,
+        // which cannot reliably decode HEVC/AV1/VP9. Advertise only codecs the
+        // WebView can decode so Jellyfin transcodes anything else to h264 HLS.
+        // (Audio-only files still direct-play via MPV, but the PlaybackInfo
+        // profile is shared, so we keep the broadly-supported audio codecs.)
+        #[cfg(all(not(target_os = "android"), target_os = "linux"))]
+        let (video_codecs, audio_codecs) = (
+            "h264".to_string(),
+            "aac,mp3,opus,vorbis,flac".to_string(),
+        );
+
+        #[cfg(all(not(target_os = "android"), not(target_os = "linux")))]
         let (video_codecs, audio_codecs) = (
             "h264,hevc,vp8,vp9,av1,mpeg4".to_string(),
             "aac,mp3,opus,vorbis,flac".to_string(),