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/index.ts

@@ -0,0 +1,4 @@
+export { Button, buttonVariants, type ButtonProps } from './button';
+export { Input, type InputProps } from './input';
+export { Card, CardHeader, CardFooter, CardTitle, CardDescription, CardContent } from './card';
+export { Label } from './label';