Add mobile QR receive payload validation

[El-swaggerito]

Description

Difficulty: Hard

Type: Feature

Summary

Add validation for QR receive payload generation.

Current Behaviour

Receive QR screens may generate payloads without validating address, network, requested amount, memo, or unsupported fields.

Expected Behaviour

The mobile app should validate QR receive payloads before display.

Suggested Implementation

Create QR payload validation, UI error states, fixtures, tests, and docs.

Files or Areas Likely Affected

• src/qr/
• src/wallet/
• src/screens/
• src/components/
• tests/
• docs/

Acceptance Criteria

• QR receive payload validation is implemented.
• Address and network are validated.
• Optional amount and memo are validated where supported.
• Invalid payload states are shown clearly.
• Tests cover valid and invalid payloads.
• Docs explain QR payload assumptions.

Additional Notes

This supports safer receive flows.