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/applicant-nav.tsx

@@ -12,6 +12,9 @@ export function ApplicantNav({ user }: ApplicantNavProps) {
   const { data: flags } = trpc.applicant.getNavFlags.useQuery(undefined, {
     staleTime: 60_000,
   })
+  const { data: featureFlags } = trpc.settings.getFeatureFlags.useQuery(undefined, {
+    staleTime: 60_000,
+  })
 
   const navigation: NavItem[] = [
     { name: 'Dashboard', href: '/applicant', icon: Home },
@@ -33,6 +36,7 @@ export function ApplicantNav({ user }: ApplicantNavProps) {
       roleName="Applicant"
       user={user}
       basePath="/applicant"
+      helpEmail={featureFlags?.supportEmail || undefined}
     />
   )
 }