Auto-load calibration.json as default intrinsics

[nikolareljin]

ID: KF-006

calibration.py saves calibration.json to a user-specified path.
capture.py only uses it if --intrinsics is explicitly set. Users who run
calibration get no benefit unless they remember to set the flag.

Fix: in capture_frames(), if intrinsics parameter is None, check for
calibration.json in Path.cwd() and load it automatically. In the GUI Capture
tab, show a status badge ("Calibration: loaded from calibration.json" or
"Calibration: using defaults") so the user can see which intrinsics are active.

Files: src/kinect_forge/capture.py, src/kinect_forge/gui.py