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/jury-nav.tsx

@@ -46,6 +46,9 @@ export function JuryNav({ user }: JuryNavProps) {
     undefined,
     { refetchInterval: 60000 }
   )
+  const { data: flags } = trpc.settings.getFeatureFlags.useQuery()
+
+  const useExternal = flags?.learningHubExternal && flags.learningHubExternalUrl
 
   const navigation: NavItem[] = [
     {
@@ -69,8 +72,9 @@ export function JuryNav({ user }: JuryNavProps) {
       : []),
     {
       name: 'Learning Hub',
-      href: '/jury/learning',
+      href: useExternal ? flags.learningHubExternalUrl : '/jury/learning',
       icon: BookOpen,
+      external: !!useExternal,
     },
   ]