docs: document environment variables

[terminalchai]

Summary

• Add a README environment-variable reference table covering backend and frontend variables read by the app.
• Add DATA_FILE to the root .env.example.
• Add frontend/.env.example for VITE_API_URL.

Verification

• rg -o process\.env\.[A-Z0-9_]+|import\.meta\.env\.[A-Z0-9_]+ -S . | Sort-Object -Unique
• npm test
• git diff --check

Closes #12

[vercel]

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