Add basic SIWE message construction helper

[Lakes41]

Difficulty: Intermediate
Type: Feature
Recommended labels (if available in this repo): enhancement, auth, sdk

Background

The roadmap notes a planned 'SIWE (Sign-In with Ethereum) signature helper.' guildpass-integrations already implements SIWE (EIP-4361) admin authentication client-side, but duplicates message construction logic locally instead of using a shared SDK helper.

Problem

There is no shared, tested SIWE message-building utility in the SDK, so each consumer (dashboard, mobile) has to hand-roll EIP-4361 message formatting, risking inconsistent or malformed messages across apps.

Expected Outcome

A createSiweMessage(params) helper in the SDK that builds a spec-compliant EIP-4361 message string given domain, address, statement, uri, version, chainId, and nonce.

Suggested Implementation

Add a new src/auth/siwe.ts module (or extend src/eip712/ if preferred, though SIWE is a plain-text message spec, not EIP-712 typed data) implementing message construction per EIP-4361's field ordering and formatting rules. Keep signature verification out of scope for this issue — this is message construction only.

Acceptance Criteria

• createSiweMessage() produces output matching the EIP-4361 message format exactly (field order and formatting)
• Function is fully typed with a SiweMessageParams interface
• Unit tests cover required fields, optional fields (statement, resources), and nonce inclusion
• pnpm test:run and pnpm typecheck pass

Likely Affected Files/Directories

• src/auth/siwe.ts
• src/eip712/eip712.types.ts

[kosisochukwu1234]

I am confident that I can implement this having worked on something similar in the past

[grantfox-oss]

🦊 GrantFox — @kosisochukwu1234 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #419)
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