Dispose email and password controllers in the login screen

[sanjidbillah]

This PR ensures proper disposal of the TextEditingController instances for the email and password fields in the login screen to prevent memory leaks.

Pre-launch Checklist

• [x] I read the [Flutter Style Guide] recently, and have followed its advice.
• [x] I signed the [CLA].
• [x] I read the [Contributors Guide].
• [x] I have added sample code updates to the [changelog].
• [x] I updated/added relevant documentation (doc comments with ///).