Implement mobile address book duplicate detection

[El-swaggerito]

Description

Difficulty: Hard

Type: Feature

Summary

Implement duplicate detection for mobile address book entries.

Current Behaviour

Users may be able to save duplicate or conflicting contacts without clear warnings.

Expected Behaviour

The mobile app should detect duplicate addresses and conflicting names before saving address book entries.

Suggested Implementation

Create duplicate detection utilities, UI warnings, storage handling, tests, and docs.

Files or Areas Likely Affected

• src/address-book/
• src/storage/
• src/screens/
• src/components/
• tests/
• docs/

Acceptance Criteria

• Duplicate address detection is implemented.
• Conflicting contact names are handled.
• User-facing warnings are shown.
• Storage remains consistent.
• Tests cover duplicate and non-duplicate cases.
• Docs explain address book rules.

Additional Notes

This improves wallet usability.