feat(auth): manage Google OAuth credentials from the database (epic 6b)

- Build the Google OAuth client lazily from sysconfig (DB override, env fallback) and re-register
  when the credentials change, so the install wizard / admin can set them without a restart.
- New 'google' config group (google_client_id/secret, encrypted) on the Configuration page; the
  token-refresh reads the creds the same way.
- Public GET /auth/config exposes google_enabled; the login page hides 'Continue with Google'
  when Google OAuth isn't configured (email+password still works).

frontend/src/components/Welcome.tsx

@@ -243,6 +243,13 @@ function AuthCard() {
   const [password, setPassword] = useState("");
   const [busy, setBusy] = useState(false);
   const [error, setError] = useState("");
+  // Whether to offer "Continue with Google" — hidden when the instance has no Google OAuth
+  // configured. Optimistic (most instances have it); the rare Google-less instance just sees it
+  // disappear once the config loads.
+  const [googleEnabled, setGoogleEnabled] = useState(true);
+  useEffect(() => {
+    api.authConfig().then((c) => setGoogleEnabled(c.google_enabled)).catch(() => {});
+  }, []);
   const [done, setDone] = useState<string>(""); // success/info message replacing the form
 
   // Explicit demo entry (replaces the old hidden probe): a labelled email field, still gated
@@ -423,12 +430,14 @@ function AuthCard() {
                 {t("welcome.auth.or")}
                 <span className="h-px flex-1 bg-border" />
               </div>
-              <a
-                href="/auth/login"
-                className="block text-center px-4 py-2.5 rounded-xl text-sm font-semibold bg-card border border-border hover:border-accent transition"
-              >
-                {t("welcome.auth.google")}
-              </a>
+              {googleEnabled && (
+                <a
+                  href="/auth/login"
+                  className="block text-center px-4 py-2.5 rounded-xl text-sm font-semibold bg-card border border-border hover:border-accent transition"
+                >
+                  {t("welcome.auth.google")}
+                </a>
+              )}
               {!showDemo ? (
                 <button
                   onClick={() => setShowDemo(true)}