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/package.json

@@ -15,8 +15,10 @@
     "lucide-react": "^0.563.0",
     "react": "^19.2.4",
     "react-dom": "^19.2.4",
+    "react-markdown": "^9.0.1",
     "react-router-dom": "^7.13.0",
-    "recharts": "^2.15.4"
+    "recharts": "^2.15.4",
+    "remark-gfm": "^4.0.1"
   },
   "devDependencies": {
     "@tailwindcss/vite": "^4.1.18",