feat(docs): add CopyButton component

Reusable copy-to-clipboard button with animated check on success.
Used by CodeBlock for all three modes.

docs/components/copy-button.tsx

@@ -0,0 +1,64 @@
+"use client";
+
+import { useState } from "react";
+
+type Props = {
+  text: string;
+  className?: string;
+};
+
+export function CopyButton({ text, className = "" }: Props) {
+  const [copied, setCopied] = useState(false);
+
+  const onClick = async () => {
+    try {
+      await navigator.clipboard.writeText(text);
+      setCopied(true);
+      setTimeout(() => setCopied(false), 1500);
+    } catch {
+      // Older browsers or denied permission — fail silently
+    }
+  };
+
+  return (
+    <button
+      type="button"
+      onClick={onClick}
+      aria-label={copied ? "Copied" : "Copy code"}
+      className={`inline-flex items-center justify-center w-7 h-7 rounded-md transition-all hover:bg-white/5 ${className}`}
+    >
+      {copied ? (
+        <svg
+          width="14"
+          height="14"
+          viewBox="0 0 24 24"
+          fill="none"
+          stroke="currentColor"
+          strokeWidth="2.4"
+          strokeLinecap="round"
+          strokeLinejoin="round"
+          className="text-emerald-400"
+          aria-hidden="true"
+        >
+          <polyline points="20 6 9 17 4 12" />
+        </svg>
+      ) : (
+        <svg
+          width="13"
+          height="13"
+          viewBox="0 0 24 24"
+          fill="none"
+          stroke="currentColor"
+          strokeWidth="2"
+          strokeLinecap="round"
+          strokeLinejoin="round"
+          className="opacity-70"
+          aria-hidden="true"
+        >
+          <rect x="9" y="9" width="13" height="13" rx="2" ry="2" />
+          <path d="M5 15H4a2 2 0 0 1-2-2V4a2 2 0 0 1 2-2h9a2 2 0 0 1 2 2v1" />
+        </svg>
+      )}
+    </button>
+  );
+}