Authentication via external providers (Google, GitHub, etc.)

[mrsumanbiswas]

Allow users to sign up / log in using OAuth2 providers in addition to the existing password method.

Backend

• Add endpoints for OAuth2 redirect/callback (e.g., GET /api/auth/google, GET /api/auth/google/callback)
• Create a new user or link account if email matches an existing user
• Issue the same JWT token pair after successful OAuth

Frontend

• Add “Continue with Google” (and others) buttons on the login/register screens
• Use platform‑specific OAuth flow or a library like google_sign_in for Flutter

Start with Google, and design the backend to easily add more providers later.