fix(calls): ship LiveKit fallback in Element Call config

Well-known fetch fails CORS on IPv4 (Wix redirect), so the embedded
widget needs the JWT service URL in its own config.json.

apps/web/res/element-call-config.json

@@ -0,0 +1,10 @@
+{
+    "matrix_rtc_session": {
+        "wait_for_key_rotation_ms": 5000,
+        "delayed_leave_event_restart_ms": 4000,
+        "delayed_leave_event_delay_ms": 18000
+    },
+    "livekit": {
+        "livekit_service_url": "https://matrix.healthchat.ch/livekit-jwt-service"
+    }
+}