feat: add copy button on code blocks

Hover over any fenced code block to reveal a floating copy-to-clipboard
button (top-right corner). Provides visual feedback (checkmark) on success.
Uses a custom ReactMarkdown <pre> component wrapper.

src/components/CodeBlock.tsx

@@ -0,0 +1,38 @@
+import { useState, useCallback, type HTMLAttributes } from 'react';
+import { Check, Copy } from 'lucide-react';
+
+/**
+ * Custom <pre> renderer for ReactMarkdown.
+ * Wraps code blocks with a floating copy button.
+ */
+export function CodeBlock(props: HTMLAttributes<HTMLPreElement>) {
+  const [copied, setCopied] = useState(false);
+
+  const handleCopy = useCallback(() => {
+    // Extract text from the nested <code> element
+    const code = (props.children as any)?.props?.children;
+    if (typeof code === 'string') {
+      navigator.clipboard.writeText(code).then(() => {
+        setCopied(true);
+        setTimeout(() => setCopied(false), 2000);
+      });
+    }
+  }, [props.children]);
+
+  return (
+    <div className="group/code relative">
+      <pre {...props} />
+      <button
+        onClick={handleCopy}
+        className="absolute top-2 right-2 p-1.5 rounded-lg bg-zinc-700/60 hover:bg-zinc-600/80 border border-white/10 text-zinc-400 hover:text-zinc-200 opacity-0 group-hover/code:opacity-100 transition-opacity duration-150"
+        title="Copy code"
+        type="button"
+      >
+        {copied
+          ? <Check className="h-3.5 w-3.5 text-green-400" />
+          : <Copy className="h-3.5 w-3.5" />
+        }
+      </button>
+    </div>
+  );
+}