chore: restructure project into backend and frontend folders

- Move all frontend code to frontend/ directory
- Add backend/ with Go project structure (cmd, internal, pkg)
- Add docker-compose.yml for orchestration

frontend/src/components/ui/Card.tsx

@@ -0,0 +1,22 @@
+import type { ReactNode } from 'react';
+
+interface CardProps {
+  children: ReactNode;
+  className?: string;
+  padding?: 'none' | 'sm' | 'md' | 'lg';
+}
+
+export const Card = ({ children, className = '', padding = 'md' }: CardProps) => {
+  const paddings = {
+    none: '',
+    sm: 'p-3',
+    md: 'p-4',
+    lg: 'p-6',
+  };
+  
+  return (
+    <div className={`bg-white rounded-lg shadow-sm border border-[#e4e2e4] ${paddings[padding]} ${className}`}>
+      {children}
+    </div>
+  );
+};