Fix teacher passwords stored in plain text

[YLaxmikanth]

Related Issue

Closes #27

Summary

Implemented secure password hashing for teacher authentication using bcryptjs.

Changes Made

• Added bcryptjs dependency for password hashing.
• Hash teacher passwords during registration.
• Validate passwords using bcrypt comparison during login.
• Added automatic migration for existing plaintext passwords.
• Updated authentication flow to support asynchronous password operations.

Security Improvements

• Passwords are no longer stored in plain text in localStorage.
• Existing users are migrated automatically after successful login.
• Backward compatibility is preserved.
• Credentials are protected using bcrypt hashing with salt rounds.

Testing

• Verified new registrations store bcrypt hashes instead of plaintext passwords.
• Verified login works correctly with hashed passwords.
• Verified legacy plaintext accounts are automatically migrated to hashed passwords after successful authentication.
• Verified production build passes (npm run build).
• Verified lint passes (npm run lint).

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