Fix Settings crash: temporal dead zone on checkDdnsStatus

checkDdnsStatus was declared via useCallback at line ~526 but referenced
in a useEffect dependency array at line 419 — before its declaration.
JavaScript const/let are not hoisted; accessing them before declaration
throws a ReferenceError (temporal dead zone). In the production build
this surfaced as:

  ReferenceError: Cannot access 'Pn' before initialization

and caused the Settings page to crash blank on load.

Moved the checkDdnsStatus useCallback definition to immediately before
the useEffect that lists it as a dependency.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

webui/src/pages/Settings.jsx

@@ -416,6 +416,18 @@ function Settings() {
   useEffect(() => { loadAll(); }, [loadAll]);
   useEffect(() => () => clearInterval(certPollRef.current), []);
 
+  const checkDdnsStatus = useCallback(async () => {
+    setDdnsStatusLoading(true);
+    try {
+      const res = await ddnsAPI.getStatus();
+      setDdnsStatus(res.data);
+    } catch {
+      setDdnsStatus(null);
+    } finally {
+      setDdnsStatusLoading(false);
+    }
+  }, []);
+
   useEffect(() => {
     if (domainMode === 'pic_ngo') checkDdnsStatus();
   }, [domainMode, checkDdnsStatus]); // eslint-disable-line react-hooks/exhaustive-deps
@@ -523,18 +535,6 @@ function Settings() {
     }
   }, [ddnsCfToken, domainName]);
 
-  const checkDdnsStatus = useCallback(async () => {
-    setDdnsStatusLoading(true);
-    try {
-      const res = await ddnsAPI.getStatus();
-      setDdnsStatus(res.data);
-    } catch {
-      setDdnsStatus(null);
-    } finally {
-      setDdnsStatusLoading(false);
-    }
-  }, []);
-
   const startCertPolling = useCallback(() => {
     clearInterval(certPollRef.current);
     setCertAcquiring(true);