[FEATURE] Add QR code support for profile sharing

[developer-ainative]

Summary

The data model (datamodel.md) specifies a qr_code field in UserProfile as a v1.1+ feature, but it is not yet implemented.

Scope

• Add QR code generation on the backend (encode profile data, store in ZeroDB file storage)
• Add QR display on the profile screen
• Add QR scanning on the discovery screen as an alternative to BLE peer discovery
• Add qr_code field to the Profile type

References

• Data model: datamodel.md line 19
• Profile type: mobile-frontend/types/profile.ts

Refs #13