Harden desktop capture constraints to prevent malformed getUserMedia errors

[Copilot]

The recorder fails to start when getUserMedia receives mixed constraint styles (legacy mandatory + standard constraints), triggering a malformed constraint error. This ensures desktop capture constraints are expressed in a single, compatible form.

• Desktop capture constraints
  • Use mandatory-only Chrome desktop constraints for preview and recording
  • Align max frame rate with the shared constant to avoid divergence

video: {
  mandatory: {
    chromeMediaSource: 'desktop',
    chromeMediaSourceId: selectedSource.id,
    maxWidth: clampedWidth,
    maxHeight: clampedHeight,
    maxFrameRate: TARGET_FRAME_RATE
  }
}

[Copilot]

Pull request overview

This PR addresses recorder start failures caused by passing mixed getUserMedia constraint styles (legacy mandatory + standard constraints). It hardens desktop-capture constraints by expressing them in a single, Chromium-compatible “mandatory-only” form for both preview and recording flows.

Changes:

• Updated preview getUserMedia video constraints to use mandatory.maxWidth/maxHeight/maxFrameRate only.
• Updated recording videoConstraints to the same mandatory-only shape to avoid malformed constraint errors.
• Standardized max frame rate to the shared TARGET_FRAME_RATE constant to prevent divergence.