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/commands/player/mod.rs

@@ -411,9 +411,22 @@ pub async fn player_play_item(
     }
 
     let controller = player.0.lock().await;
+    // On Linux, video plays in the WebKitGTK HTML5 <video> element (see
+    // get_player_status -> use_html5_element). The MPV backend has no embedded
+    // window, so loading the stream into it would only start a redundant decode
+    // (and the frontend would immediately stop it). Only load into the native
+    // backend on platforms that actually render video through it (e.g. Android).
+    #[cfg(not(target_os = "linux"))]
     controller
         .play_item(media_item)
         .map_err(|e| e.to_string())?;
+    #[cfg(target_os = "linux")]
+    {
+        // Keep the queue in sync for UI/remote-transfer without starting MPV.
+        controller
+            .set_current_item(media_item)
+            .map_err(|e| e.to_string())?;
+    }
 
     // Emit queue changed event
     controller.emit_queue_changed();