fix(frontend): require public env vars at build time

[willchen96]

Summary

• fail production frontend builds when required public connection variables are missing or blank
• validate the Supabase URL, Supabase publishable key, and backend API URL together
• document that NEXT_PUBLIC_* values must be present during next build, not only at runtime

This prevents deploying a browser bundle that cannot connect to Supabase or the backend API.

Closes #92.
Addresses the frontend build-time configuration portion of #265.

Verification

• missing-value production build fails with the expected list of all three variables
• configured npm run build --prefix frontend passes
• npm test --prefix frontend — 47 passed
• npm run lint --prefix frontend — 0 errors (existing warnings remain)