feat: Page Editor v1.0 - canonical schema, SSR parity, and migration

Major improvements to WooNooW Page Editor system:

Schema & Architecture:
- Canonical section schema with unified sectionSchema.ts
- Normalized feature-grid to use items (not features)
- Standardized default values across all section types
- Schema versioning with automatic migration on read

Backend (PHP):
- Enhanced PlaceholderRenderer with typed output contracts
- Added fallback behavior for empty/invalid dynamic sources
- Added caching support for post data resolution
- New SchemaMigration class for backward compatibility
- New Features class for feature flags
- Enhanced PageSSR with full style support
- Removed controller-level special-casing for related_posts

Frontend (Admin SPA):
- Updated CanvasRenderer with schema-aware transformation
- Enhanced InspectorPanel with canonical schema metadata
- Added new section renderers

Frontend (Customer SPA):
- New section components: BentoCategoryGrid, MarqueeBanner, ProductCarousel, ShoppableImage
- Updated FeatureGridSection for items prop contract

Testing:
- Add PHP tests: SchemaMigrationTest, PlaceholderRendererTest, PageSSRTest
- Add TypeScript tests: schema-integration, feature-grid-regression
- Add parity tests for React vs SSR content matching
- Add CI script: check-schema-drift.mjs
- Add VERIFICATION_CHECKLIST.md

Documentation:
- RELEASE_NOTES-v1.0.md with full release notes
- docs/PAGE_EDITOR_SECTION_SCHEMA_V1.md
- docs/PAGE_EDITOR_SSR_COVERAGE_AUDIT.md

customer-spa/src/styles/theme.css

@@ -189,8 +189,25 @@
  * DARK MODE
  * ======================================== */
 
+:root.dark {
+  --color-background: #1F2937;
+  --color-text: #F9FAFB;
+
+  /* Invert gray scale for dark mode */
+  --color-gray-50: #111827;
+  --color-gray-100: #1F2937;
+  --color-gray-200: #374151;
+  --color-gray-300: #4B5563;
+  --color-gray-400: #6B7280;
+  --color-gray-500: #9CA3AF;
+  --color-gray-600: #D1D5DB;
+  --color-gray-700: #E5E7EB;
+  --color-gray-800: #F3F4F6;
+  --color-gray-900: #F9FAFB;
+}
+
 @media (prefers-color-scheme: dark) {
-  :root {
+  :root:not(.light):not(.dark) {
     --color-background: #1F2937;
     --color-text: #F9FAFB;
 
@@ -328,4 +345,4 @@ img {
   .container {
     max-width: 1536px;
   }
-}
+}