feat: external Learning Hub toggle + applicant help button

- Add admin settings: learning_hub_external, learning_hub_external_url, support_email
- Jury/Mentor nav respects external Learning Hub URL (opens in new tab)
- RoleNav supports external nav items with ExternalLink icon
- Applicant header shows Help button with configurable support email
- Settings update mutation now upserts (creates on first use)
- Shared inferSettingCategory for consistent category assignment

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

src/components/layouts/mentor-nav.tsx

@@ -2,12 +2,17 @@
 
 import { BookOpen, Home, Users } from 'lucide-react'
 import { RoleNav, type NavItem, type RoleNavUser } from '@/components/layouts/role-nav'
+import { trpc } from '@/lib/trpc/client'
 
 interface MentorNavProps {
   user: RoleNavUser
 }
 
 export function MentorNav({ user }: MentorNavProps) {
+  const { data: flags } = trpc.settings.getFeatureFlags.useQuery()
+
+  const useExternal = flags?.learningHubExternal && flags.learningHubExternalUrl
+
   const navigation: NavItem[] = [
     {
       name: 'Dashboard',
@@ -21,8 +26,9 @@ export function MentorNav({ user }: MentorNavProps) {
     },
     {
       name: 'Learning Hub',
-      href: '/mentor/resources',
+      href: useExternal ? flags.learningHubExternalUrl : '/mentor/resources',
       icon: BookOpen,
+      external: !!useExternal,
     },
   ]