Replace hardcoded `localhost:8080` URLs with a centralised Axios instance

`NotesPage.jsx` and `login.jsx` both hardcode `http://localhost:8080` in every Axios call. This makes it impossible to change the backend URL without editing multiple files.
Create a centralised `frontend/src/api.js` that exports a configured Axios instance using `VITE_API_BASE_URL` from `.env`.
Replace all raw `axios.get/post/put/delete` calls in `NotesPage.jsx` and `login.jsx` with the new instance.
- Touches: `frontend/src/NotesPage.jsx`, `frontend/src/login.jsx`, new `frontend/src/api.js`, `frontend/.env`

Start by reading frontend/src/NotesPage.jsx and frontend/src/login.jsx to locate every raw Axios call and the existing localhost:8080 URLs. Then review frontend/.env and create the central client in frontend/src/api.js using VITE_API_BASE_URL. Done means both named components use the shared instance and no longer hardcode the backend URL.

Written by the indexing model from the issue text.