remove the onboarding flow and the accountMode leftover

Onboarding was dead in three layers:

- The OnboardingAdmin screen was only reachable from a route block in App.tsx
  that has been commented out, so it never rendered. Its ServerTypeCard carried
  accountMode ('organization' | 'single'), inherited from the codebase this was
  based on and meaningless for a single-user platform.
- Two /onboarding-complete endpoints, one public and one protected, that no
  frontend code called. Both read a server_config key that was never written, so
  both answered false while the app's own path defaulted to true.
- HomeScreen gated on settings.onboarding.complete to show a welcome panel, and
  seedHomeDir created an Onboarding folder from DATA_PATH/Onboarding and
  /Onboarding_Admin — neither seed directory exists, so it only ever produced an
  empty folder.

Also drops the onboarding key from UserSettings and the now-empty home-header
panel from the default home layout.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/servers/api/server-settings/server-settings.ts

@@ -26,11 +26,6 @@ serverSettingsRouter.get('/settings', async (ctx) => {
   return ctx.json(await readServerSettings());
 });
 
-serverSettingsRouter.get('/onboarding-complete', async (ctx) => {
-  const settings = await readServerSettings();
-  return ctx.json({ onboardingComplete: !!settings.onboardingComplete });
-});
-
 serverSettingsRouter.put('/', async (ctx) => {
   const body = await ctx.req.json();
   const settings = await readServerSettings();