Feature: Improve & refactor the Login page

## Description
In #185, we noticed that the codebase of the current Login page is still very ugly and non-nullsafety.
Hence, we should migrate it to null-safety, and refactor it to a better practice. 

## How to implement?
Just refactor the whole page.

## Design(optional) 
- TBD

## Additional contexts(optional)
- TBD
