Error message are not descriptive enough on login/signup

[niat-web]

Bug Report

What happened:
When login fails (wrong password) or signup fails
(email already exists), the error messages
shown to the user are not clear or helpful.

Expected behavior:
Login failure:
"Invalid email or password. Please check and try again."

Signup with existing email:
"An account with this email already exists.
Please login instead."

Password too weak:
"Password must be at least 8 characters and
include uppercase, lowercase, number and special character."

Steps to reproduce:

1. Go to login page
2. Enter wrong password
3. See vague error message

Why this is important:
Clear error messages help users understand
what went wrong and how to fix it.