Add transparent background query param for Vision Pro pass-through

visionOS Safari doesn't support immersive-ar WebXR sessions. Pass-through
on Vision Pro requires immersive-vr with a transparent scene background.
When display_mode is pass-through, pass background=none query param to Vuer
client, which (with corresponding JS patch) skips the opaque scene background.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/televuer/televuer.py

@@ -88,7 +88,10 @@ class TeleVuer:
                     cert_file = cert_file or str(current_module_dir / "cert.pem")
                     key_file = key_file or str(current_module_dir / "key.pem")
 
-        self.vuer = Vuer(host='0.0.0.0', cert=cert_file, key=key_file, queries=dict(grid=False), queue_len=3)
+        queries = dict(grid=False)
+        if display_mode == "pass-through":
+            queries["background"] = "none"
+        self.vuer = Vuer(host='0.0.0.0', cert=cert_file, key=key_file, queries=queries, queue_len=3)
         self.vuer.add_handler("CAMERA_MOVE")(self.on_cam_move)
         if self.use_hand_tracking:
             self.vuer.add_handler("HAND_MOVE")(self.on_hand_move)