Add mobile wallet import error states

[El-swaggerito]

Description

Difficulty: Expert

Type: Feature

Summary

Add robust error states for wallet import screens.

Current Behaviour

Wallet import failures may not be clearly represented for invalid secret, empty input, duplicate wallet, secure-store failure, or cancellation.

Expected Behaviour

The mobile app should show safe, helpful, and non-sensitive wallet import errors.

Suggested Implementation

Implement error mapping, UI states, secure-store failure handling, tests, and docs.

Files or Areas Likely Affected

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

Acceptance Criteria

• Wallet import error states are implemented.
• Invalid and empty input states are handled.
• Secure-store failure state is handled.
• Sensitive values are never shown.
• Tests cover major failure states.
• Docs explain wallet import error handling.

Additional Notes

This is security-sensitive mobile work.