fix: use generic error messages in login handler (#845)

[Shrutii-Rai]

**Fixes #845

📝 Description

Replaced specific authentication error messages with generic messages to prevent user enumeration attacks.**

## 🔗 Related Issue
Closes #845

## 🛠️ Changes Made

• Replaced "User does not exist" with "Invalid email or password"
• Replaced "Password does not match" with "Invalid email or password"
• Prevents attackers from knowing whether email is registered or password is wrong

✅ Checklist

• Followed project structure
• Linked the issue
• No console errors
• Only authController.js modified (clean fix)

🚀 Notes for Reviewers

This is a resubmission of PR #866 which was closed due to accidental code deletion during conflict resolution. This PR contains only the intended 2-line message changes.

[Shrutii-Rai]

Hi @aryandas2911

I'm submitting this as a clean resubmission of PR #866 (which was
closed due to accidental code deletion during conflict resolution —
I apologize for that mistake!).

## What was the bug? 🐛
The login handler was returning specific error messages:

• "User does not exist" → revealed whether an email is registered
• "Password does not match" → revealed the email was valid but password was wrong

This is a User Enumeration Attack vulnerability — attackers can
use specific messages to figure out which emails are registered in
the system, and then target those accounts.

## What did I fix?
Replaced both specific messages with a single generic message:

• ✅ "Invalid email or password"

This prevents attackers from knowing whether the email exists or
the password was wrong — both cases return the same message.

## Why is this important?
This is a standard security best practice followed by major platforms
like Google, GitHub, and others. It significantly reduces the risk of
targeted attacks on user accounts.

## Changes Made 📝

• Only backend/controllers/authController.js was modified
• 2 error messages changed in the login handler
• No other files touched — clean minimal fix ✅

Kindly review and merge this PR when you get a chance.
Happy to make any changes if needed! 🙏

Thank you!

[Shrutii-Rai]

Hi @aryandas2911
Could you please review and merge PR #1177 when you get a chance?
I have also been assigned issue #1181 and would like to start working on it.
Thank you!

[aryandas2911]

@Shrutii-Rai there are no changed files in this PR

[aryandas2911]

@Shrutii-Rai there are no changed files in this PR