- Add /qr/<uuid> endpoint to generate QR code images server-side
- Replace client-side JavaScript QR generation with reliable server-side PNG generation
- Add qrcode and Pillow dependencies to requirements.txt
- Remove JavaScript QR code library dependency
- QR codes now display reliably in all browsers

The QR code authentication flow now works properly:
1. User clicks 'Link Mobile App' → Server generates UUID and QR code image
2. QR code displays immediately (no JavaScript dependency)
3. Mobile app scans QR code → Extracts URL with UUID
4. POST to /mobileapp?uuid={uuid} → Receives API token
5. Automatic authentication and app access

Fixes the issue where QR codes weren't displaying due to JavaScript library loading problems.