feat: implement onboarding wizard and fix help page navigation

Core Features:
- Add Quick Setup Wizard for new users with multi-step flow
- Implement distraction-free onboarding layout (no sidebar/header)
- Create OnboardingController API endpoint for saving settings
- Redirect new users to /setup automatically on first admin access

Onboarding Components:
- StepMode: Select between full/minimal store modes
- StepHomepage: Choose or auto-create homepage
- StepAppearance: Configure container width and primary color
- StepProgress: Visual progress indicator

Navigation & Routing:
- Fix Help page links to use react-router navigation (prevent full reload)
- Update onboarding completion redirect to /appearance/pages
- Add manual onboarding access via Settings > Store Details

UI/UX Improvements:
- Enable dark mode support for Page Editor
- Fix page title rendering in onboarding dropdown
- Improve title fallback logic (title.rendered, title, post_title)

Type Safety:
- Unify PageItem interface across all components
- Add 'default' to containerWidth type definition
- Add missing properties (permalink_base, has_template, icon)

Files Modified:
- includes/Api/OnboardingController.php
- includes/Api/Routes.php
- includes/Admin/Assets.php
- admin-spa/src/App.tsx
- admin-spa/src/routes/Onboarding/*
- admin-spa/src/routes/Help/DocContent.tsx
- admin-spa/src/routes/Settings/Store.tsx
- admin-spa/src/routes/Appearance/Pages/*

admin-spa/src/routes/Appearance/Pages/components/InspectorPanel.tsx

@@ -32,7 +32,7 @@ import {
 import { InspectorField, SectionProp } from './InspectorField';
 import { InspectorRepeater } from './InspectorRepeater';
 import { MediaUploader } from '@/components/MediaUploader';
-import { SectionStyles, ElementStyle } from '../store/usePageEditorStore';
+import { SectionStyles, ElementStyle, PageItem } from '../store/usePageEditorStore';
 
 interface Section {
     id: string;
@@ -44,17 +44,6 @@ interface Section {
     props: Record<string, SectionProp>;
 }
 
-interface PageItem {
-    id?: number;
-    type: 'page' | 'template';
-    cpt?: string;
-    slug?: string;
-    title: string;
-    url?: string;
-    isSpaLanding?: boolean;
-    containerWidth?: 'boxed' | 'fullwidth';
-}
-
 interface InspectorPanelProps {
     page: PageItem | null;
     selectedSection: Section | null;