fix(web): repair 6 dead internal links + harden forum markdown against stored XSS

Routing: footer "Explorer"/"Validators" pointed to non-existent /network (4 spots
in AppLayout + LandingPageDesktop) and Explorer/Forum quick-links pointed to bare
/governance (no route) -> all hit the 404 catch-all. Repointed to /explorer and
/governance/assembly; footer "Vote" /-> /elections.

Security (stored XSS): DiscussionThread.parseMarkdown did regex->HTML then
dangerouslySetInnerHTML with no escaping/sanitizer, so <img onerror>/javascript:
links in user comments executed. Now escape HTML first, then DOMPurify-sanitize the
output to a safe tag/attr allow-list with http(s)-only hrefs.

web/src/pages/Explorer.tsx

@@ -860,7 +860,7 @@ const Explorer: React.FC = () => {
                   {t('explorer.telemetry')}
                 </a>
                 <a
-                  href="/governance"
+                  href="/governance/assembly"
                   className="flex items-center gap-2 p-3 rounded-lg bg-gray-800 hover:bg-gray-750 text-gray-300 hover:text-white transition-colors"
                 >
                   <Users className="w-4 h-4 text-blue-400" />

web/src/pages/Forum.tsx

@@ -429,7 +429,7 @@ const Forum: React.FC = () => {
                   </CardTitle>
                 </CardHeader>
                 <CardContent className="space-y-2">
-                  <a href="/governance" className="block text-gray-400 hover:text-green-400 text-sm transition-colors">
+                  <a href="/governance/assembly" className="block text-gray-400 hover:text-green-400 text-sm transition-colors">
                     → {t('forum.govDashboard')}
                   </a>
                   <a href="/docs" className="block text-gray-400 hover:text-green-400 text-sm transition-colors">