feat: add dynamic meta tags for social sharing (Phase 4-5)

Phase 4: Dynamic Meta Tags
- Added react-helmet-async dependency
- Created SEOHead component with Open Graph and Twitter Card support
- Added HelmetProvider wrapper to App.tsx
- Integrated SEOHead in Product page (title, description, image, product info)
- Integrated SEOHead in Shop page (basic meta tags)

Phase 5: Auto-Flush Permalinks
- Enhanced settings change handler to only flush when spa_mode,
  spa_page, or use_browser_router changes
- Plugin already flushes on activation (Installer.php)

This enables proper link previews when sharing product URLs
on Facebook, Twitter, Slack, etc.

customer-spa/package.json

@@ -30,6 +30,7 @@
         "lucide-react": "^0.547.0",
         "react": "^18.3.1",
         "react-dom": "^18.3.1",
+        "react-helmet-async": "^2.0.5",
         "react-hook-form": "^7.54.2",
         "react-router-dom": "^7.9.4",
         "sonner": "^2.0.7",