feat: Implement foundational UI components (Button, Card) and layout components (Section, Container), updating design tokens, styling, and documentation.

components/ui/Card.tsx

@@ -0,0 +1,21 @@
+import type { ElementType, ReactNode } from "react";
+
+interface CardProps {
+  children: ReactNode;
+  className?: string;
+  as?: ElementType;
+}
+
+export function Card({
+  children,
+  className = "",
+  as: Tag = "div",
+}: CardProps) {
+  return (
+    <Tag
+      className={`rounded-[var(--radius-md)] border border-[color:var(--color-border)] shadow-[var(--shadow-sm)] p-[var(--spacing-lg)] ${className}`.trim()}
+    >
+      {children}
+    </Tag>
+  );
+}