fix(docs): client-side nonce in SIWE example is insecure — add server-side nonce warning

## Problem

File: docs/mini-apps/quickstart/migrate-to-standard-web-app.mdx
Section: "Replace auth and identity" → SignIn.tsx code example

The example uses `generateSiweNonce()` on the client side:

const nonce = generateSiweNonce();

A client-generated nonce provides no replay protection. Any attacker who
intercepts a valid SIWE message+signature pair can reuse it, because the
server has no record of which nonces it issued.

## Current behavior

The nonce is generated in the browser and never validated server-side.
The existing disclaimer is easy to miss and does not explain why the
current pattern is unsafe.

## Expected behavior

Add a prominent warning block above the code example explaining that
client-side nonces offer no replay protection, OR replace the example
with a server-issued nonce pattern.

## Impact

Developers following this migration guide verbatim will ship apps with
broken replay protection, exposing users to signature replay attacks.

## References

- SIWE spec: https://eips.ethereum.org/EIPS/eip-4361
- Related fix merged in this repo: #1398