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/Tabs.tsx

@@ -0,0 +1,59 @@
+import * as React from 'react'
+import * as TabsPrimitive from '@radix-ui/react-tabs'
+import { cn } from '../../utils/cn'
+
+const Tabs = TabsPrimitive.Root
+
+const TabsList = React.forwardRef<
+  React.ElementRef<typeof TabsPrimitive.List>,
+  React.ComponentPropsWithoutRef<typeof TabsPrimitive.List>
+>(({ className, ...props }, ref) => (
+  <TabsPrimitive.List
+    ref={ref}
+    className={cn(
+      'inline-flex h-10 items-center justify-center rounded-lg',
+      'bg-surface-subtle p-1 text-content-muted',
+      className
+    )}
+    {...props}
+  />
+))
+TabsList.displayName = TabsPrimitive.List.displayName
+
+const TabsTrigger = React.forwardRef<
+  React.ElementRef<typeof TabsPrimitive.Trigger>,
+  React.ComponentPropsWithoutRef<typeof TabsPrimitive.Trigger>
+>(({ className, ...props }, ref) => (
+  <TabsPrimitive.Trigger
+    ref={ref}
+    className={cn(
+      'inline-flex items-center justify-center whitespace-nowrap',
+      'rounded-md px-3 py-1.5 text-sm font-medium',
+      'ring-offset-surface-base transition-all duration-fast',
+      'focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-brand-500 focus-visible:ring-offset-2',
+      'disabled:pointer-events-none disabled:opacity-50',
+      'data-[state=active]:bg-surface-elevated data-[state=active]:text-content-primary data-[state=active]:shadow-sm',
+      className
+    )}
+    {...props}
+  />
+))
+TabsTrigger.displayName = TabsPrimitive.Trigger.displayName
+
+const TabsContent = React.forwardRef<
+  React.ElementRef<typeof TabsPrimitive.Content>,
+  React.ComponentPropsWithoutRef<typeof TabsPrimitive.Content>
+>(({ className, ...props }, ref) => (
+  <TabsPrimitive.Content
+    ref={ref}
+    className={cn(
+      'mt-2 ring-offset-surface-base',
+      'focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-brand-500 focus-visible:ring-offset-2',
+      className
+    )}
+    {...props}
+  />
+))
+TabsContent.displayName = TabsPrimitive.Content.displayName
+
+export { Tabs, TabsList, TabsTrigger, TabsContent }