Add Social Login to Lovable App

Add Google and GitHub social login to my Lovable app using Supabase Auth.

REQUIREMENTS:
1. Add "Continue with Google" and "Continue with GitHub" buttons on the Sign In and Sign Up pages with proper brand icons and colors.
2. Configure Supabase Auth to use Google and GitHub as OAuth providers.
3. On successful OAuth callback, check if a profile exists in the "profiles" table. If not, auto-create one using the OAuth user data (name, avatar, email).
4. Handle the OAuth redirect flow — redirect to /auth/callback, process the session, then redirect to the dashboard.
5. Show a loading state during the OAuth redirect.
6. If the OAuth email matches an existing email/password account, link the accounts.
7. Handle errors gracefully: provider unavailable, user denied access, email already registered.

DATABASE:
- No new tables. Uses existing "profiles" table.
- Update the profile creation trigger to handle OAuth metadata.

ENVIRONMENT VARIABLES:
- Google: GOOGLE_CLIENT_ID, GOOGLE_CLIENT_SECRET (set in Supabase Dashboard)
- GitHub: GITHUB_CLIENT_ID, GITHUB_CLIENT_SECRET (set in Supabase Dashboard)

SECURITY:
- Configure redirect URLs in both Google/GitHub OAuth app settings and Supabase.
- Only allow redirects to your verified domain.