fix: normalize email inputs to lowercase in signup, login, and googleLogin (fixes #982)

[SamraddhiJoshi]

Description

This PR resolves issue #982:

• Normalizes email inputs by trimming whitespace and converting them to lowercase during signup, login, and googleLogin processes inside authController.js.
• This ensures case-insensitive database lookups and prevents duplicate accounts with different casings.

Tests and Verification

• Ran backend unit tests successfully.
• Verified ESLint rules pass without warnings.