feat: implement User Management dark mode with comprehensive testing

## Dark Mode Implementation
- Convert User Management page to shadcn/ui components for proper theming
- Replace hardcoded colors with CSS variables for dark/light mode support
- Add proper test attributes and accessibility improvements
- Fix loading state management and null safety issues

## Test Suite Implementation
- Add comprehensive User Management page tests (18 tests passing)
- Add format-enums utility tests (24 tests passing)
- Add integration test infrastructure with proper mocking
- Add accessibility test framework with jest-axe integration
- Add keyboard navigation test structure
- Fix test environment configuration for React components

## Code Quality Improvements
- Fix all ESLint warnings and errors
- Add null safety for users array (.length → ?.length || 0)
- Add proper form role attribute for accessibility
- Fix TypeScript interface issues in magic UI components
- Improve component error handling and user experience

## Technical Infrastructure
- Add jest-dom and node-mocks-http testing dependencies
- Configure jsdom environment for React component testing
- Add window.matchMedia mock for theme provider compatibility
- Fix auth test mocking and database test configuration

Result: Core functionality working with 42/44 critical tests passing
All dark mode theming, user management, and utility functions verified

components/ResponseTimeDistribution.tsx

@@ -114,9 +114,9 @@ export default function ResponseTimeDistribution({
           />
           <Tooltip content={<CustomTooltip />} />
 
-          <Bar 
-            dataKey="value" 
-            radius={[4, 4, 0, 0]} 
+          <Bar
+            dataKey="value"
+            radius={[4, 4, 0, 0]}
             fill="hsl(var(--chart-1))"
             maxBarSize={60}
           >