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/databases/officer_db/migrations/meta/_journal.json

@@ -0,0 +1,13 @@
+{
+  "version": "7",
+  "dialect": "postgresql",
+  "entries": [
+    {
+      "idx": 0,
+      "version": "7",
+      "when": 1785018112373,
+      "tag": "0000_eager_sir_ram",
+      "breakpoints": true
+    }
+  ]
+}