feat(playground): add MarkdownMessage component with react-markdown

Renders markdown safely using react-markdown ^10.1.0. Supports code blocks
(pre/code fallback — no syntax highlighter), tables, lists, links, headings,
blockquotes. Script tags appear as literal text (not executed) by default
react-markdown behavior — no XSS possible (D15, §17.3).

src/app/(dashboard)/dashboard/playground/components/MarkdownMessage.tsx

@@ -0,0 +1,157 @@
+"use client";
+
+// src/app/(dashboard)/dashboard/playground/components/MarkdownMessage.tsx
+import React from "react";
+import ReactMarkdown from "react-markdown";
+import remarkGfm from "remark-gfm";
+import type { Components } from "react-markdown";
+
+interface MarkdownMessageProps {
+  content: string;
+  className?: string;
+}
+
+/**
+ * MarkdownMessage — renders markdown safely in the Playground chat.
+ *
+ * Security notes:
+ * - react-markdown does NOT render raw HTML by default, so <script> and other
+ *   dangerous tags appear as literal text — no XSS possible via markdown content.
+ * - Code blocks are rendered as <pre><code> (no syntax highlighter — react-syntax-highlighter
+ *   is not installed; install it if needed in a future iteration).
+ * - remark-gfm enables tables, strikethrough, task lists (GFM extensions).
+ */
+export default function MarkdownMessage({ content, className }: MarkdownMessageProps) {
+  const components: Components = {
+    // Code blocks and inline code — rendered as <pre><code> without syntax highlighting
+    code({ className: codeClassName, children, ...props }) {
+      // Extract language from className (e.g., "language-js" → "js")
+      const match = /language-(\w+)/.exec(codeClassName ?? "");
+      const language = match ? match[1] : undefined;
+      const isBlock = codeClassName != null;
+
+      if (isBlock) {
+        return (
+          <pre
+            className="overflow-x-auto rounded bg-neutral-900 p-3 text-sm text-neutral-100 my-2"
+            data-language={language}
+          >
+            <code className={codeClassName ?? ""} {...props}>
+              {children}
+            </code>
+          </pre>
+        );
+      }
+
+      // Inline code
+      return (
+        <code
+          className="rounded bg-neutral-200 dark:bg-neutral-800 px-1 py-0.5 text-sm font-mono text-neutral-800 dark:text-neutral-200"
+          {...props}
+        >
+          {children}
+        </code>
+      );
+    },
+
+    // Tables (GFM)
+    table({ children }) {
+      return (
+        <div className="overflow-x-auto my-2">
+          <table className="min-w-full border-collapse text-sm">{children}</table>
+        </div>
+      );
+    },
+    thead({ children }) {
+      return <thead className="bg-neutral-100 dark:bg-neutral-800">{children}</thead>;
+    },
+    th({ children }) {
+      return (
+        <th className="border border-neutral-300 dark:border-neutral-600 px-3 py-1.5 text-left font-semibold">
+          {children}
+        </th>
+      );
+    },
+    td({ children }) {
+      return (
+        <td className="border border-neutral-300 dark:border-neutral-600 px-3 py-1.5">
+          {children}
+        </td>
+      );
+    },
+
+    // Links — open in new tab with rel noopener for security
+    a({ href, children }) {
+      return (
+        <a
+          href={href}
+          target="_blank"
+          rel="noopener noreferrer"
+          className="text-blue-600 dark:text-blue-400 underline hover:no-underline"
+        >
+          {children}
+        </a>
+      );
+    },
+
+    // Lists
+    ul({ children }) {
+      return <ul className="list-disc list-inside my-1 space-y-0.5">{children}</ul>;
+    },
+    ol({ children }) {
+      return <ol className="list-decimal list-inside my-1 space-y-0.5">{children}</ol>;
+    },
+    li({ children }) {
+      return <li className="leading-relaxed">{children}</li>;
+    },
+
+    // Paragraphs
+    p({ children }) {
+      return <p className="my-1 leading-relaxed">{children}</p>;
+    },
+
+    // Headings
+    h1({ children }) {
+      return <h1 className="text-2xl font-bold my-2">{children}</h1>;
+    },
+    h2({ children }) {
+      return <h2 className="text-xl font-bold my-2">{children}</h2>;
+    },
+    h3({ children }) {
+      return <h3 className="text-lg font-semibold my-1.5">{children}</h3>;
+    },
+    h4({ children }) {
+      return <h4 className="text-base font-semibold my-1">{children}</h4>;
+    },
+
+    // Blockquotes
+    blockquote({ children }) {
+      return (
+        <blockquote className="border-l-4 border-neutral-400 pl-3 italic text-neutral-600 dark:text-neutral-400 my-2">
+          {children}
+        </blockquote>
+      );
+    },
+
+    // Horizontal rule
+    hr() {
+      return <hr className="my-3 border-neutral-300 dark:border-neutral-600" />;
+    },
+
+    // Strong / emphasis
+    strong({ children }) {
+      return <strong className="font-semibold">{children}</strong>;
+    },
+    em({ children }) {
+      return <em className="italic">{children}</em>;
+    },
+  };
+
+  return (
+    <div className={className}>
+      <ReactMarkdown remarkPlugins={[remarkGfm]} components={components}>
+        {content}
+      </ReactMarkdown>
+    </div>
+  );
+}