docs: add React Router middleware example

[aqilaziz]

What changed

• Adds a React Router v7 Framework Mode middleware example to the README.
• Shows how to wire createServerClient with getAll and setAll using request cookies and response Set-Cookie headers.
• Shows how to expose the Supabase client and verified user via React Router context.

Fixes #149

Verification

• git diff --check

Reference: https://reactrouter.com/how-to/middleware