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