feat: add Google Sign-In to login and register pages

- Add GoogleOAuthProvider wrapper in layout.tsx
- Add Google login button to LoginForm with "or continue with email" separator
- Add Google signup button to RegisterForm
- Add Google auth types and API client integration
- Add GOOGLE_CLIENT_ID and NEXT_PUBLIC_GOOGLE_CLIENT_ID to env config
- Add translations (EN/FR) for Google OAuth UI
- Backend already has /api/v1/auth/google endpoint, no changes needed

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.env.production

@@ -25,6 +25,10 @@ TRANSLATION_SERVICE=google
 # ---- Google (gratuit, toujours actif) ----
 GOOGLE_TRANSLATE_ENABLED=true
 
+# ---- Google OAuth (connexion avec Google) ----
+GOOGLE_CLIENT_ID=
+NEXT_PUBLIC_GOOGLE_CLIENT_ID=
+
 # ---- Ollama (local, gratuit) ----
 OLLAMA_ENABLED=false
 OLLAMA_BASE_URL=http://ollama:11434