Complete Settings page and fix TypeScript issues

- Create comprehensive SettingsPage with General, Storage, and Security tabs
- Add formatBytes method to settings service
- Update AdminSidebar to show real storage usage from backend
- Fix TypeScript errors with react-query v5 (isPending instead of isLoading)
- Remove unused imports and fix type imports
- Add Settings route to App.tsx
- Implement real-time storage monitoring in sidebar

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

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

frontend/src/App.tsx

@@ -17,7 +17,8 @@ import {
   EmailConfigPage,
   ArchivesPage,
   AnalyticsPage,
-  BrandingPage
+  BrandingPage,
+  SettingsPage
 } from './pages/admin';
 import { AdminLayout } from './components/admin';
 import { PageErrorBoundary, ErrorBoundary, OfflineIndicator, SkipLink } from './components/common';
@@ -76,6 +77,7 @@ function App() {
                     <Route path="email" element={<EmailConfigPage />} />
                     <Route path="analytics" element={<AnalyticsPage />} />
                     <Route path="branding" element={<BrandingPage />} />
+                    <Route path="settings" element={<SettingsPage />} />
                     <Route path="/" element={<Navigate to="/admin/dashboard" replace />} />
                   </Route>
                 </Routes>