ESLint warnings cleanup and frontend code quality improvements

While running the EduAid frontend locally, several ESLint warnings are reported during development.
These include unused variables, equality checks using `==` instead of `===`, missing dependencies in
`useEffect` hooks, and a few naming/structure-related warnings.

Although these warnings do not currently break functionality, they increase noise during development
and can make onboarding new contributors more difficult.

This issue proposes a focused cleanup to:
- resolve existing ESLint warnings
- improve frontend code readability and consistency
- keep all runtime behavior and API interactions unchanged

The intention is to strictly limit the scope to linting and code-quality improvements, without
introducing functional changes.