feat(auth): sign in with Google behind NEXT_PUBLIC_GOOGLE_AUTH_ENABLED (#1441)

Adds a 'Continue with Google' button to login and register, gated by
NEXT_PUBLIC_GOOGLE_AUTH_ENABLED so it ships dark until the Google
provider is configured in Supabase. The OAuth round-trip reuses the
existing /auth/callback PKCE exchange, which already owns MFA routing,
invite acceptance and silent-team creation. A flow=oauth marker on the
redirect lets the callback tag failures (including provider consent
denials) so the login page shows Google-specific error copy instead of
the email-confirmation framing.

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

messages/en.json

@@ -274,6 +274,9 @@
     "login_error_rate_limited": "Too many sign-in attempts. Wait a moment and try again.",
     "login_error_user_banned": "This account has been suspended. Contact support if you think this is a mistake.",
     "login_error_reset_link": "Reset your password",
+    "continue_with_google": "Continue with Google",
+    "callback_error_title_oauth": "Google sign-in didn't work",
+    "callback_error_body_oauth": "The Google sign-in could not be completed. Try again, or sign in with email and password.",
     "session_idle": "You were inactive. Sign in again.",
     "session_absolute": "Your session expired for security reasons.",
     "use_password_instead": "Sign in with email instead",

messages/sv.json

@@ -274,6 +274,9 @@
     "login_error_rate_limited": "För många inloggningsförsök. Vänta en stund och försök igen.",
     "login_error_user_banned": "Kontot är avstängt. Kontakta supporten om du tror att det är fel.",
     "login_error_reset_link": "Återställ lösenordet",
+    "continue_with_google": "Fortsätt med Google",
+    "callback_error_title_oauth": "Google-inloggningen fungerade inte",
+    "callback_error_body_oauth": "Inloggningen med Google kunde inte slutföras. Försök igen, eller logga in med e-post och lösenord.",
     "session_idle": "Du har varit inaktiv. Logga in igen.",
     "session_absolute": "Sessionen har upphört av säkerhetsskäl.",
     "use_password_instead": "Logga in med e-post i stället",