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/TechniqueDetailPage.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,
@@ -182,9 +183,10 @@ export default function TechniqueDetailPage() {
         {/* Description */}
         <div className="lg:col-span-2 rounded-xl border border-gray-800 bg-gray-900 p-6">
           <h2 className="mb-3 text-lg font-semibold text-white">Description</h2>
-          <p className="text-sm text-gray-400 leading-relaxed whitespace-pre-wrap">
-            {technique.description || "No description available."}
-          </p>
+          <MarkdownText
+            content={technique.description || "No description available."}
+            className="text-sm text-gray-400 leading-relaxed"
+          />
         </div>
 
         {/* Metadata */}
@@ -579,7 +581,7 @@ function DetectionRulesSection({ rules }: { rules: DetectionRule[] }) {
                     onClick={(e) => e.stopPropagation()}
                   >
                     {rule.description && (
-                      <p className="text-xs text-gray-400 leading-relaxed">{rule.description}</p>
+                      <MarkdownText content={rule.description} className="text-xs text-gray-400 leading-relaxed" />
                     )}
                     <div className="flex flex-wrap gap-3 text-xs text-gray-500">
                       {rule.rule_format && (
@@ -702,7 +704,7 @@ function D3FENDSection({ defenses }: { defenses: Array<{
                   {isExpanded && (
                     <div className="mt-3 space-y-2 border-t border-gray-700/50 pt-3">
                       {def.description ? (
-                        <p className="text-xs text-gray-300 leading-relaxed">{def.description}</p>
+                        <MarkdownText content={def.description} className="text-xs text-gray-300 leading-relaxed" />
                       ) : (
                         <p className="text-xs text-gray-500 italic">No description available.</p>
                       )}