fix: impersonation logout, applicant learning hub redirect, nav click tracking

- Sign Out button during impersonation now returns to admin instead of
  destroying the session (fixes multi-click logout issue)
- Applicant nav now respects learning_hub_external setting like jury/mentor
- Track Learning Hub nav clicks via audit log (LEARNING_HUB_CLICK action)

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

src/components/layouts/applicant-nav.tsx

@@ -16,6 +16,8 @@ export function ApplicantNav({ user }: ApplicantNavProps) {
     staleTime: 60_000,
   })
 
+  const useExternal = featureFlags?.learningHubExternal && featureFlags.learningHubExternalUrl
+
   const navigation: NavItem[] = [
     { name: 'Dashboard', href: '/applicant', icon: Home },
     { name: 'Project', href: '/applicant/team', icon: FolderOpen },
@@ -27,7 +29,12 @@ export function ApplicantNav({ user }: ApplicantNavProps) {
     ...(flags?.hasMentor
       ? [{ name: 'Mentoring', href: '/applicant/mentor', icon: MessageSquare }]
       : []),
-    { name: 'Resources', href: '/applicant/resources', icon: BookOpen },
+    {
+      name: 'Learning Hub',
+      href: useExternal ? featureFlags.learningHubExternalUrl : '/applicant/resources',
+      icon: BookOpen,
+      external: !!useExternal,
+    },
   ]
 
   return (