feat(frontend): render markdown in description and summary fields

- New shared MarkdownText component (react-markdown + remark-gfm)
  that renders links, bold, italic, lists, code, blockquotes.
  External links open in a new tab with rel=noopener.
- Applied to: technique description, threat actor description,
  test description, campaign description, detection rule descriptions,
  D3FEND defense descriptions, red/blue summaries and validation notes.
- procedure_text (code/commands) stays in <pre> — not processed as MD.

frontend/src/pages/TestDetailPage.tsx

@@ -1,4 +1,5 @@
 import { useParams, useNavigate } from "react-router-dom";
+import MarkdownText from "../components/MarkdownText";
 import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query";
 import { useState, useEffect, useCallback } from "react";
 import { Loader2, AlertCircle, ArrowLeft } from "lucide-react";
@@ -441,8 +442,8 @@ export default function TestDetailPage() {
             <dl className="space-y-4">
               <div>
                 <dt className="text-xs font-medium uppercase text-gray-500">Description</dt>
-                <dd className="mt-1 text-sm text-gray-300">
-                  {test.description || "\u2014"}
+                <dd className="mt-1">
+                  <MarkdownText content={test.description || "\u2014"} className="text-sm text-gray-300" />
                 </dd>
               </div>
               <div>