fix: React hooks rules violation in Login.tsx causing crash on load

useCallback hooks were placed AFTER conditional early returns.
When authLoading toggled from true -> false, the hook count changed
between renders (14 hooks vs 17 hooks), triggering React's
"Rendered more hooks than during the previous render" error.

Moved all useCallback definitions before the conditional returns.

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

src/admin/pages/Login.tsx

@@ -43,20 +43,6 @@ export default function Login() {
     }
   }, [show2FA, useBackupCode]);
 
-  if (authLoading) {
-    return (
-      <div className="admin-login">
-        <div className="admin-loading">
-          <div className="admin-spinner" />
-        </div>
-      </div>
-    );
-  }
-
-  if (isAuthenticated && !animatingOut) {
-    return <Navigate to="/" replace />;
-  }
-
   const handleSubmit = useCallback(
     async (e: React.FormEvent) => {
       e.preventDefault();
@@ -143,6 +129,20 @@ export default function Login() {
     setUseBackupCode(false);
   }, [setShow2FA, setLoginToken, setTotpCode, setUseBackupCode]);
 
+  if (authLoading) {
+    return (
+      <div className="admin-login">
+        <div className="admin-loading">
+          <div className="admin-spinner" />
+        </div>
+      </div>
+    );
+  }
+
+  if (isAuthenticated && !animatingOut) {
+    return <Navigate to="/" replace />;
+  }
+
   return (
     <motion.div
       className="admin-login"