QR camera scanner (GC0308 + quirc)

[kwsantiago]

Implement QR code scanning for air-gapped PSBT and policy input.

Hardware

• Camera: GC0308 0.3MP (VGA, DVP interface)
• Output: RGB565 (requires grayscale conversion)

Tasks

• Add esp32-camera and quirc dependencies
• Implement qr_scanner_init/scan/deinit
• RGB565→grayscale conversion for quirc
• Detect QR types: PSBT, UR, address, policy
• Integrate with ux_display_backend.scan_qr()
• Camera preview on display while scanning

QR Type Detection

• Base64 PSBT (cHNidP8...)
• UR format (ur:crypto-psbt/...)
• Bitcoin addresses (bc1q, bc1p, 1..., 3...)
• Policy bundles (JSON with warden_pubkey)

Performance Targets

Operation	Target
QR scan	< 3s
Grayscale conversion	< 20ms
Frame rate	~33 FPS

Dependencies

• Requires: Display + touch (Display and touch interface (LVGL + CoreS3) #20)