Upgrade Vite 8 and @vitejs/plugin-react 6

[shigahi]

Summary

• Upgrade vite from 7.3.1 to 8.0.0 (Rolldown bundler, lightningcss built-in, Babel removed)
• Upgrade @vitejs/plugin-react from 5.1.4 to 6.0.0 (Babel dependency removed, uses Oxc for React Refresh)
• Combined upgrade required due to mutual peer dependency constraints
• Replaces Dependabot PRs Bump vite from 7.3.1 to 8.0.0 #74 and Bump @vitejs/plugin-react from 5.1.4 to 6.0.0 #75

Test plan

• npm install succeeds with 0 vulnerabilities
• npm run build succeeds (built in 1.29s)
• Verify dev server works (npm start)
• Verify deployed Workers site works (npm run dev)