feat: make frontend backend API URL configurable via environment variable

[Rahul-2k4]

Summary

The frontend api.ts hardcodes the backend API URL as http://127.0.0.1:5000/, which prevents deployment to hosted environments where the backend runs at a different origin (e.g., PythonAnywhere).

This PR makes the URL configurable via the NEXT_PUBLIC_API_BASE_URL environment variable, falling back to the local default when unset.

Related to #8 – deployment readiness.

Changes

• frontend/services/api.ts: Read NEXT_PUBLIC_API_BASE_URL from process.env with fallback to http://127.0.0.1:5000/
• frontend/.env.example: Document the new variable
• frontend/.gitignore: Explicitly list env files (instead of broad .env* glob) so .env.example can be committed

Usage

Local development (default -- no action needed):

npm run dev

Point at a deployed backend:

cp frontend/.env.example frontend/.env.local
# Edit NEXT_PUBLIC_API_BASE_URL in .env.local
npm run dev