Frontend: Add OAuth Login Buttons [93-6]

[mehalyna]

• Task: Add social login buttons (Google, GitHub) to the frontend login page.
• Steps:
  1. Create buttons: "Login with Google" and "Login with GitHub".
  2. Implement logic to redirect to the OAuth provider.
  3. Handle OAuth callback to process authentication.
• Deliverable: OAuth buttons and login redirection implemented.

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US #93