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/api/routes.py

@@ -126,7 +126,7 @@ async def status():
         "color_available": _camera.is_color_available() if (_camera and _camera.is_open()) else False,
         "usb_type": _camera.usb_type() if (_camera and _camera.is_open()) else None,
         "frame_count": _frame_count,
-        "resolution": {"width": 640, "height": 480},
+        "resolution": _camera.resolution() if (_camera and _camera.is_open()) else None,
         "intrinsics": _calibration_payload(),
     })