Add first-run setup wizard

Full-screen onboarding at admin.php?page=aurora-admin-setup: Welcome →
Appearance (theme + font) → Branding (logo/favicon/rename) → Features
(replacement-screen toggles) → Done. Reuses SettingsField for every control
and applyLiveTheme for instant preview; finish/skip saves via wp/v2/settings
and marks completion.

- admin/src/Pages/SetupWizard.php: hidden submenu page, disables the shell on
  its own page (aurora_admin_shell_enabled filter) + hides native chrome,
  register_activation_hook sets a one-time redirect flag, admin_init redirects
  once, and a POST /aurora-admin/v1/setup/complete route marks it done.
- app/src/pages/setup-wizard/SetupWizard.vue + entries/setup-wizard.js + vite
  input.
- Wired into aurora-admin.php + AuroraAdmin.php.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

app/src/entries/setup-wizard.js

@@ -0,0 +1,10 @@
+import { createApp } from 'vue';
+import '@/shell/shell-frame.css';
+import SetupWizard from '@/pages/setup-wizard/SetupWizard.vue';
+
+const mountEl = document.getElementById('aurora-admin-setup-root');
+
+if (mountEl) {
+  const data = JSON.parse(mountEl.dataset.auroraAdmin || '{}');
+  createApp(SetupWizard, { serverData: data }).mount(mountEl);
+}