Add user authentication

[swathiiie]

Implemented basic user authentication system.

✔ Added user registration and login functionality
✔ Added session persistence using localStorage
✔ Implemented per-user note storage (notes are isolated by username)
✔ Added logout functionality

Each user's notes are now securely separated and accessible only after successful login.

Closes #46

[rspavithra]

I noticed there is a merge conflict in this PR.
Kindly resolve the conflict and push the updated changes

[rspavithra]

After reviewing and running the project, the following issues were observed:

The login page is not appearing as expected when the app loads.
The authentication flow does not seem to function as described.
The logout button is visible, but it is not working properly.
An image import option has been added, which was not part of the assigned issue.
There are multiple errors/warnings in script.js (showing 9+ issues), which need to be resolved.

Kindly update the PR accordingly.