feat: implement modern UI/UX design system (#409)

- Add comprehensive design token system (colors, typography, spacing)
- Create 12 new UI components with Radix UI primitives
- Add layout components (PageShell, StatsCard, EmptyState, DataTable)
- Polish all pages with new component library
- Improve accessibility with WCAG 2.1 compliance
- Add dark mode support with semantic color tokens
- Update 947 tests to match new UI patterns

Closes #409

frontend/src/components/ui/Button.tsx

@@ -1,55 +1,110 @@
-import { ButtonHTMLAttributes, ReactNode } from 'react'
-import { clsx } from 'clsx'
+import * as React from 'react'
+import { cva, type VariantProps } from 'class-variance-authority'
+import { Loader2, type LucideIcon } from 'lucide-react'
+import { cn } from '../../utils/cn'
 
-interface ButtonProps extends ButtonHTMLAttributes<HTMLButtonElement> {
-  variant?: 'primary' | 'secondary' | 'danger' | 'ghost'
-  size?: 'sm' | 'md' | 'lg'
+const buttonVariants = cva(
+  [
+    'inline-flex items-center justify-center gap-2',
+    'rounded-lg font-medium',
+    'transition-all duration-fast',
+    'focus:outline-none focus-visible:ring-2 focus-visible:ring-offset-2 focus-visible:ring-offset-surface-base',
+    'disabled:opacity-50 disabled:cursor-not-allowed disabled:pointer-events-none',
+  ],
+  {
+    variants: {
+      variant: {
+        primary: [
+          'bg-brand-500 text-white',
+          'hover:bg-brand-600',
+          'focus-visible:ring-brand-500',
+          'active:bg-brand-700',
+        ],
+        secondary: [
+          'bg-surface-muted text-content-primary',
+          'hover:bg-surface-subtle',
+          'focus-visible:ring-content-muted',
+          'active:bg-surface-base',
+        ],
+        danger: [
+          'bg-error text-white',
+          'hover:bg-error/90',
+          'focus-visible:ring-error',
+          'active:bg-error/80',
+        ],
+        ghost: [
+          'text-content-secondary bg-transparent',
+          'hover:bg-surface-muted hover:text-content-primary',
+          'focus-visible:ring-content-muted',
+        ],
+        outline: [
+          'border border-border bg-transparent text-content-primary',
+          'hover:bg-surface-subtle hover:border-border-strong',
+          'focus-visible:ring-brand-500',
+        ],
+        link: [
+          'text-brand-500 bg-transparent underline-offset-4',
+          'hover:underline hover:text-brand-400',
+          'focus-visible:ring-brand-500',
+          'p-0 h-auto',
+        ],
+      },
+      size: {
+        sm: 'h-8 px-3 text-sm',
+        md: 'h-10 px-4 text-sm',
+        lg: 'h-12 px-6 text-base',
+        icon: 'h-10 w-10 p-0',
+      },
+    },
+    defaultVariants: {
+      variant: 'primary',
+      size: 'md',
+    },
+  }
+)
+
+export interface ButtonProps
+  extends React.ButtonHTMLAttributes<HTMLButtonElement>,
+    VariantProps<typeof buttonVariants> {
   isLoading?: boolean
-  children: ReactNode
+  leftIcon?: LucideIcon
+  rightIcon?: LucideIcon
+  asChild?: boolean
 }
 
-export function Button({
-  variant = 'primary',
-  size = 'md',
-  isLoading = false,
-  className,
-  children,
-  disabled,
-  ...props
-}: ButtonProps) {
-  const baseStyles = 'inline-flex items-center justify-center rounded-lg font-medium transition-colors focus:outline-none focus:ring-2 focus:ring-offset-2 disabled:opacity-50 disabled:cursor-not-allowed'
-
-  const variants = {
-    primary: 'bg-blue-600 text-white hover:bg-blue-700 focus:ring-blue-500',
-    secondary: 'bg-gray-700 text-white hover:bg-gray-600 focus:ring-gray-500',
-    danger: 'bg-red-600 text-white hover:bg-red-700 focus:ring-red-500',
-    ghost: 'text-gray-400 hover:text-white hover:bg-gray-800 focus:ring-gray-500',
+const Button = React.forwardRef<HTMLButtonElement, ButtonProps>(
+  (
+    {
+      className,
+      variant,
+      size,
+      isLoading = false,
+      leftIcon: LeftIcon,
+      rightIcon: RightIcon,
+      disabled,
+      children,
+      ...props
+    },
+    ref
+  ) => {
+    return (
+      <button
+        className={cn(buttonVariants({ variant, size }), className)}
+        ref={ref}
+        disabled={disabled || isLoading}
+        {...props}
+      >
+        {isLoading ? (
+          <Loader2 className="h-4 w-4 animate-spin" />
+        ) : (
+          LeftIcon && <LeftIcon className="h-4 w-4" />
+        )}
+        {children}
+        {!isLoading && RightIcon && <RightIcon className="h-4 w-4" />}
+      </button>
+    )
   }
+)
+Button.displayName = 'Button'
 
-  const sizes = {
-    sm: 'px-3 py-1.5 text-sm',
-    md: 'px-4 py-2 text-sm',
-    lg: 'px-6 py-3 text-base',
-  }
-
-  return (
-    <button
-      className={clsx(
-        baseStyles,
-        variants[variant],
-        sizes[size],
-        className
-      )}
-      disabled={disabled || isLoading}
-      {...props}
-    >
-      {isLoading && (
-        <svg className="animate-spin -ml-1 mr-2 h-4 w-4 text-current" xmlns="http://www.w3.org/2000/svg" fill="none" viewBox="0 0 24 24">
-          <circle className="opacity-25" cx="12" cy="12" r="10" stroke="currentColor" strokeWidth="4"></circle>
-          <path className="opacity-75" fill="currentColor" d="M4 12a8 8 0 018-8V0C5.373 0 0 5.373 0 12h4zm2 5.291A7.962 7.962 0 014 12H0c0 3.042 1.135 5.824 3 7.938l3-2.647z"></path>
-        </svg>
-      )}
-      {children}
-    </button>
-  )
-}
+export { Button, buttonVariants }