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/components/Sidebar.tsx

@@ -14,7 +14,7 @@ import '../styles/Sidebar.css';
 export default function Sidebar() {
   const { t, language, setLanguage } = useTranslation();
   const { config } = useSiteConfig();
-  const { sidebarStyle, theme, toggleTheme, darkModeLocation, languageLocation, showDarkModeToggle, showLanguageToggle } = useTheme();
+  const { sidebarStyle, theme, toggleTheme, darkModeLocation, languageLocation, showDarkModeToggle, showLanguageToggle, hasInitializedSettings: themeInitialized } = useTheme();
   const {
     isCollapsed,
     isMobileOpen,
@@ -235,7 +235,7 @@ export default function Sidebar() {
             </button>
           )}
 
-          {showDarkModeToggle && darkModeLocation === 'sidebar' && (
+          {themeInitialized && showDarkModeToggle && darkModeLocation === 'sidebar' && (
             <button
               className="view-mode-toggle"
               onClick={() => { toggleTheme(); updateTooltipText(theme === 'dark' ? t.theme.lightMode : t.theme.darkMode); }}
@@ -254,7 +254,7 @@ export default function Sidebar() {
             </button>
           )}
 
-          {showLanguageToggle && languageLocation === 'sidebar' && (
+          {themeInitialized && showLanguageToggle && languageLocation === 'sidebar' && (
             <button
               className="view-mode-toggle"
               onClick={() => { setLanguage(language === 'it' ? 'en' : 'it'); updateTooltipText(language === 'it' ? t.settings.english : t.settings.italian); }}