Add email validation and improvements for login/signup

[Ismailco]

Description:
We need to improve the login and signup forms by adding proper checks and validations.

Tasks:

• Add email format validation (frontend + backend)
• Ensure unique email check during signup (backend)
• Add password strength validation (min length, complexity)
• Show clear error messages for invalid input
• Sanitize inputs to prevent injection
• Add loading state for form submission
• Improve overall form UX (disable button until valid, etc.)

Expected Outcome:
Users should not be able to sign up or log in with invalid or duplicate emails, weak passwords, or unsanitized input.