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.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/ThreatActorDetailPage.tsx

@@ -1,4 +1,5 @@
 import { useParams, useNavigate } from "react-router-dom";
+import MarkdownText from "../components/MarkdownText";
 import { useQuery } from "@tanstack/react-query";
 import {
   Loader2,
@@ -263,9 +264,7 @@ export default function ThreatActorDetailPage() {
           <h2 className="mb-3 text-sm font-semibold uppercase tracking-wider text-gray-500">
             Description
           </h2>
-          <p className="text-sm leading-relaxed text-gray-300 whitespace-pre-line">
-            {actor.description}
-          </p>
+          <MarkdownText content={actor.description} className="text-sm leading-relaxed text-gray-300" />
         </div>
       )}