feat: add CopyButton client component

thoughts-frontend/components/copy-button.tsx

@@ -0,0 +1,33 @@
+"use client";
+
+import { useState } from "react";
+import { Check, Copy } from "lucide-react";
+import { cn } from "@/lib/utils";
+
+interface CopyButtonProps {
+  text: string;
+  className?: string;
+}
+
+export function CopyButton({ text, className }: CopyButtonProps) {
+  const [copied, setCopied] = useState(false);
+
+  const handleCopy = async () => {
+    await navigator.clipboard.writeText(text);
+    setCopied(true);
+    setTimeout(() => setCopied(false), 1500);
+  };
+
+  return (
+    <button
+      onClick={handleCopy}
+      title="Copy to clipboard"
+      className={cn(
+        "inline-flex items-center justify-center rounded p-0.5 text-muted-foreground/60 transition-colors hover:text-muted-foreground",
+        className
+      )}
+    >
+      {copied ? <Check className="h-3 w-3" /> : <Copy className="h-3 w-3" />}
+    </button>
+  );
+}