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>

frontend/src/messages/en.json

@@ -212,7 +212,8 @@
     },
     "login": {
       "title": "Welcome back",
-      "subtitle": "Sign in to keep translating",
+      "welcomeBack": "Welcome back",
+      "signInToContinue": "Sign in to keep translating",
       "email": "Email",
       "password": "Password",
       "emailPlaceholder": "you@example.com",
@@ -221,7 +222,8 @@
       "signIn": "Sign in",
       "errorTitle": "Sign-in error",
       "noAccount": "Don't have an account?",
-      "signUpFree": "Sign up for free"
+      "signUpFree": "Sign up for free",
+      "orContinueWith": "or continue with email"
     },
     "register": {
       "title": "Create an account",

frontend/src/messages/fr.json

@@ -212,7 +212,8 @@
     },
     "login": {
       "title": "Bon retour",
-      "subtitle": "Connectez-vous pour continuer à traduire",
+      "welcomeBack": "Bon retour",
+      "signInToContinue": "Connectez-vous pour continuer à traduire",
       "email": "E-mail",
       "password": "Mot de passe",
       "emailPlaceholder": "vous@exemple.com",
@@ -221,7 +222,8 @@
       "signIn": "Se connecter",
       "errorTitle": "Erreur de connexion",
       "noAccount": "Pas encore de compte ?",
-      "signUpFree": "Créer un compte gratuitement"
+      "signUpFree": "Créer un compte gratuitement",
+      "orContinueWith": "ou continuer avec email"
     },
     "register": {
       "title": "Créer un compte",