fix: disable login submit button until email is valid (#27)

[MsadafK]

Summary

• Derived isEmailValid state from email input in src/app/login/page.tsx (checks if email is non-empty and contains @).
• Disabled "Send login code" submit button when email is empty or invalid.
• Added keyboard submit protection to prevent form submission on invalid email.
• Added subtle helper text when typing an invalid email address.

Related issue

Fixes #27

Type of change

• Bug fix
• New feature
• Documentation
• Other (describe below)

How to test

1. Navigate to /login.
2. Observe that the "Send login code" button is disabled initially.
3. Type test (without @) → Button remains disabled and helper text appears.
4. Type test@example.com → Button becomes enabled.
5. Press Enter key on an invalid email to confirm no submission/API call happens.

Checklist

• I ran npm run build (and npm run contracts:test if contracts changed)
• I did not commit secrets or .env.local
• I read CONTRIBUTING.md

[FrankezeCode]

This is awesome , thanks 👍