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/CampaignDetailPage.tsx

@@ -1,5 +1,6 @@
 import { useState } from "react";
 import { useParams, useNavigate } from "react-router-dom";
+import MarkdownText from "../components/MarkdownText";
 import { useQuery, useMutation, useQueryClient } from "@tanstack/react-query";
 import {
   Loader2,
@@ -236,7 +237,7 @@ export default function CampaignDetailPage() {
                 </span>
               </div>
               {campaign.description && (
-                <p className="mt-1 text-sm text-gray-400">{campaign.description}</p>
+                <MarkdownText content={campaign.description} className="mt-1 text-sm text-gray-400" />
               )}
               <div className="mt-2 flex flex-wrap items-center gap-3 text-xs text-gray-500">
                 {campaign.threat_actor_name && (