feat(camera): gate calibration on webcam lighting quality check

[midaa1]

Summary

• Adds a real-time lighting quality indicator on the Camera Configuration
  screen.
• Disables Start Calibration until the webcam feed has been in a
  "good" lighting state for ~1.5 s (3 consecutive samples at 2 Hz).
• Helps users avoid calibrating under conditions that silently degrade
  gaze accuracy (too dark, backlit, or flat/low-contrast scenes).

What changed

src/views/CameraConfiguration.vue

• Samples luminance (mean + stddev) from every 10th pixel of the existing
  detectFace RAF canvas at 2 Hz — no extra frame capture.
• Classifies into good / dark / bright / lowContrast / unknown
  and renders a Vuetify chip under the preview with an icon + message.
• lightingReady computed requires 3 consecutive good samples; the
  streak resets on any non-good reading, re-disabling the button.
• startCalibration button :disabled now also checks !lightingReady.

Classification thresholds

• mean < 60 → dark
• mean > 200 → bright
• stddev < 18 → low contrast
• otherwise → good

Testing

• unstable photo :

• stable one:

References :

• luminance coefficients: https://en.wikipedia.org/wiki/Rec._601
• MDN getImageData: https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/getImageData