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/player/mod.rs

@@ -212,6 +212,22 @@ impl PlayerController {
         Ok(())
     }
 
+    /// Set the current queue item without loading it into the playback backend.
+    ///
+    /// Used on platforms where video is rendered outside the native backend
+    /// (Linux WebKitGTK HTML5 <video>): the queue/UI state must reflect the
+    /// item, but MPV must not start a redundant decode for it.
+    pub fn set_current_item(&self, item: MediaItem) -> Result<(), PlayerError> {
+        debug!("[PlayerController] set_current_item (no backend load): {}", item.title);
+
+        self.reset_autoplay_count();
+
+        let mut queue = self.queue.lock_safe();
+        queue.set_queue(vec![item], 0);
+
+        Ok(())
+    }
+
     /// Load and play an item without modifying the queue
     /// Use this when the queue is already set up and you just want to play a specific item from it
     pub fn load_and_play(&self, item: &MediaItem) -> Result<(), PlayerError> {