Add project documentation

[Yifu-Tian]

Summary

• Replace the starter-template README with Kleo Connect project documentation
• Add setup, environment variable, app flow, authentication, API, and project structure notes
• Document development commands and contribution steps

Verification

• git diff --check
• README content reviewed
• npm install completed with engine warnings on local Node v10.19.0
• npm run test -- --run was not completed because local Node v10.19.0 cannot run Vitest ESM entrypoints
• npm run build was not completed because local Node v10.19.0 cannot run Vite ESM entrypoints

Addresses #56