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/slider.tsx

@@ -0,0 +1,63 @@
+"use client";
+
+import * as React from "react";
+import * as SliderPrimitive from "@radix-ui/react-slider";
+
+import { cn } from "@/lib/utils";
+
+function Slider({
+  className,
+  defaultValue,
+  value,
+  min = 0,
+  max = 100,
+  ...props
+}: React.ComponentProps<typeof SliderPrimitive.Root>) {
+  const _values = React.useMemo(
+    () =>
+      Array.isArray(value)
+        ? value
+        : Array.isArray(defaultValue)
+          ? defaultValue
+          : [min, max],
+    [value, defaultValue, min, max]
+  );
+
+  return (
+    <SliderPrimitive.Root
+      data-slot="slider"
+      defaultValue={defaultValue}
+      value={value}
+      min={min}
+      max={max}
+      className={cn(
+        "relative flex w-full touch-none items-center select-none data-[disabled]:opacity-50 data-[orientation=vertical]:h-full data-[orientation=vertical]:min-h-44 data-[orientation=vertical]:w-auto data-[orientation=vertical]:flex-col",
+        className
+      )}
+      {...props}
+    >
+      <SliderPrimitive.Track
+        data-slot="slider-track"
+        className={cn(
+          "bg-muted relative grow overflow-hidden rounded-full data-[orientation=horizontal]:h-1.5 data-[orientation=horizontal]:w-full data-[orientation=vertical]:h-full data-[orientation=vertical]:w-1.5"
+        )}
+      >
+        <SliderPrimitive.Range
+          data-slot="slider-range"
+          className={cn(
+            "bg-primary absolute data-[orientation=horizontal]:h-full data-[orientation=vertical]:w-full"
+          )}
+        />
+      </SliderPrimitive.Track>
+      {Array.from({ length: _values.length }, (_, index) => (
+        <SliderPrimitive.Thumb
+          data-slot="slider-thumb"
+          key={index}
+          className="border-primary bg-background ring-ring/50 block size-4 shrink-0 rounded-full border shadow-sm transition-[color,box-shadow] hover:ring-4 focus-visible:ring-4 focus-visible:outline-hidden disabled:pointer-events-none disabled:opacity-50"
+        />
+      ))}
+    </SliderPrimitive.Root>
+  );
+}
+
+export { Slider };