Add SIWE wallet signature verification helpers

[Lakes41]

The roadmap includes wallet signature verification helpers. Add SIWE utilities so apps can verify wallet ownership before checking guild access.

Suggested tasks

• Add utilities for creating SIWE messages
• Add utilities for verifying SIWE signatures
• Support nonce generation or validation helpers
• Export the helpers from the SDK entry point
• Add examples for server-side verification
• Add tests for valid and invalid signatures

Acceptance criteria

• SIWE helper functions are available from the SDK
• Developers can verify wallet ownership before access checks
• Signature verification behaviour is tested
• Usage is documented

[Adesam007-pr3dator]

Hi, I would love to work on this issue

[leocagli]

I'd like to work on this — please assign me!

[grantfox-oss]

🦊 GrantFox — @Adesam007-pr3dator has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #14)
2. Your PR will be reviewed by the Adamantine-guild maintainers

Good luck! Track your progress on GrantFox.