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/ui/tabs.tsx

@@ -0,0 +1,66 @@
+"use client";
+
+import * as React from "react";
+import * as TabsPrimitive from "@radix-ui/react-tabs";
+
+import { cn } from "@/lib/utils";
+
+function Tabs({
+  className,
+  ...props
+}: React.ComponentProps<typeof TabsPrimitive.Root>) {
+  return (
+    <TabsPrimitive.Root
+      data-slot="tabs"
+      className={cn("flex flex-col gap-2", className)}
+      {...props}
+    />
+  );
+}
+
+function TabsList({
+  className,
+  ...props
+}: React.ComponentProps<typeof TabsPrimitive.List>) {
+  return (
+    <TabsPrimitive.List
+      data-slot="tabs-list"
+      className={cn(
+        "bg-muted text-muted-foreground inline-flex h-9 w-fit items-center justify-center rounded-lg p-[3px]",
+        className
+      )}
+      {...props}
+    />
+  );
+}
+
+function TabsTrigger({
+  className,
+  ...props
+}: React.ComponentProps<typeof TabsPrimitive.Trigger>) {
+  return (
+    <TabsPrimitive.Trigger
+      data-slot="tabs-trigger"
+      className={cn(
+        "data-[state=active]:bg-background dark:data-[state=active]:text-foreground focus-visible:border-ring focus-visible:ring-ring/50 focus-visible:outline-ring dark:data-[state=active]:border-input dark:data-[state=active]:bg-input/30 text-foreground dark:text-muted-foreground inline-flex h-[calc(100%-1px)] flex-1 items-center justify-center gap-1.5 rounded-md border border-transparent px-2 py-1 text-sm font-medium whitespace-nowrap transition-[color,box-shadow] focus-visible:ring-[3px] focus-visible:outline-1 disabled:pointer-events-none disabled:opacity-50 data-[state=active]:shadow-sm [&_svg]:pointer-events-none [&_svg]:shrink-0 [&_svg:not([class*='size-'])]:size-4",
+        className
+      )}
+      {...props}
+    />
+  );
+}
+
+function TabsContent({
+  className,
+  ...props
+}: React.ComponentProps<typeof TabsPrimitive.Content>) {
+  return (
+    <TabsPrimitive.Content
+      data-slot="tabs-content"
+      className={cn("flex-1 outline-none", className)}
+      {...props}
+    />
+  );
+}
+
+export { Tabs, TabsList, TabsTrigger, TabsContent };