fix: report negotiated color resolution in /status

Read width/height/fps from the live pipeline profile instead of
hardcoding 640x480, so /status reflects the firmware-accepted
resolution (e.g. the USB 2.x 640x480@15 fallback) rather than the
requested config.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/backend/services/camera.py

@@ -135,6 +135,21 @@ class RealSenseD435:
     def usb_type(self) -> str:
         return self._usb_type
 
+    def resolution(self) -> Optional[dict]:
+        """Negotiated color stream resolution, or None if not open.
+
+        Reads from the live pipeline profile so the value reflects what the
+        firmware actually accepted — on USB 2.x this is the forced 640x480@15
+        fallback, not the configured `_width/_height`.
+        """
+        if not self._open or self._profile is None:
+            return None
+        try:
+            stream = self._profile.get_stream(rs.stream.color).as_video_stream_profile()
+            return {"width": stream.width(), "height": stream.height(), "fps": stream.fps()}
+        except Exception:
+            return None
+
     def init(self) -> bool:
         try:
             self._ctx = rs.context()