feat(web): add base UI components

- Button with variants (default, destructive, outline, secondary, ghost, link, success)
- Input with consistent styling
- Card with Header, Title, Description, Content, Footer
- Label using Radix UI primitives
- Index file for centralized exports

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/web/components/ui/label.tsx

@@ -0,0 +1,25 @@
+'use client';
+
+import * as React from 'react';
+import * as LabelPrimitive from '@radix-ui/react-label';
+import { cva, type VariantProps } from 'class-variance-authority';
+import { cn } from '@/lib/utils';
+
+const labelVariants = cva(
+  'text-sm font-medium leading-none peer-disabled:cursor-not-allowed peer-disabled:opacity-70'
+);
+
+const Label = React.forwardRef<
+  React.ElementRef<typeof LabelPrimitive.Root>,
+  React.ComponentPropsWithoutRef<typeof LabelPrimitive.Root> &
+    VariantProps<typeof labelVariants>
+>(({ className, ...props }, ref) => (
+  <LabelPrimitive.Root
+    ref={ref}
+    className={cn(labelVariants(), className)}
+    {...props}
+  />
+));
+Label.displayName = LabelPrimitive.Root.displayName;
+
+export { Label };