Add UI settings API and improve context initialization

- Add /settings/ui GET and PUT endpoints for UI settings
- Improve ThemeContext with better initialization and auto accent handling
- Update SidebarContext with expanded state persistence
- Fix context initialization in ModulesContext and ViewModeContext
- Update components to use improved theme/sidebar contexts

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/pages/admin/Features.tsx

@@ -13,7 +13,7 @@ export default function Features() {
     const { user: currentUser } = useAuth();
     const { t } = useTranslation();
     const { toggleMobileMenu } = useSidebar();
-    const { moduleStates, setModuleEnabled, saveModulesToBackend, hasInitialized } = useModules();
+    const { moduleStates, setModuleEnabled, saveModulesToBackend, hasInitialized, isLoading } = useModules();
     const [activeTab, setActiveTab] = useState<TabId>('feature1');
     const hasUserMadeChanges = useRef(false);
     const saveRef = useRef(saveModulesToBackend);
@@ -119,6 +119,9 @@ export default function Features() {
     };
 
     const renderTabContent = () => {
+        if (!hasInitialized || isLoading) {
+            return <div className="loading">Loading...</div>;
+        }
         switch (activeTab) {
             case 'feature1':
                 return (