fix: whitespace search guard and unknown section fallback

- SettingsTab: use searchQuery.trim() instead of searchQuery to prevent
  whitespace-only input from triggering empty search results view
- AdminSettings: fallback to branding when activeSection is unknown
  instead of rendering blank content area

src/pages/AdminSettings.tsx

@@ -216,6 +216,8 @@ export default function AdminSettings() {
             />
           );
         }
+        // Unknown section — fallback to branding
+        setActiveSection('branding');
         return null;
     }
   };