fix: replace insecure Math.random() nonces with crypto APIs

[clicheman]

What changed? Why?

Replaced all Math.random() usages for nonce generation with cryptographically secure alternatives across three documentation files.

Math.random() is not cryptographically secure and produces predictable output. Using it for SIWE nonces or security-sensitive values exposes developers who copy these examples to replay attacks.

Changes

File	Before	After
wagmi/setup.mdx	Math.random().toString(36).substring(2, 15)	crypto.randomUUID()
sign-in-with-base-button.mdx	Math.random().toString(36).substring(7)	crypto.randomUUID()
sign-and-verify-typed-data.mdx	Math.floor(Math.random() * 1000000)	crypto.randomInt(1_000_000) + added import crypto

Notes to reviewers

• crypto.randomUUID() is available in all modern browsers and Node.js 19+ (Web Crypto API)
• crypto.randomInt() is used in the server-side example (Node.js crypto module) since the nonce there is a numeric value
• Consistent with the existing authenticate-users guide which already uses crypto.randomUUID()

Closes #1477

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/2	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2