fix: SIWE nonce passthrough from SDK config

[skgbafa]

Summary

• Bug: When the web SDK passes a nonce via siweConfig: { nonce }, the Rust WASM layer silently drops it because SessionConfig has no nonce field (serde discards unknown fields) and into_message() hardcodes nonce: generate_nonce() — always creating a random one.
• Fix: Added an optional nonce field to SessionConfig and changed the SIWE message construction to use self.nonce.unwrap_or_else(generate_nonce), so a caller-provided nonce is respected while falling back to random generation when none is provided.

Test plan

• cargo check -p tinycloud-sdk-wasm passes with no errors
• Existing tests continue to pass (no nonce provided = random nonce, same behavior as before)
• Web SDK can pass siweConfig: { nonce: "server-issued-nonce" } and verify it appears in the SIWE message