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.

includes/Frontend/TemplateOverride.php

@@ -16,10 +16,21 @@ class TemplateOverride
         // Register rewrite rules for BrowserRouter SEO (must be on 'init')
         add_action('init', [__CLASS__, 'register_spa_rewrite_rules']);
         
-        // Flush rewrite rules when appearance settings are updated
-        add_action('update_option_woonoow_appearance_settings', function() {
-            flush_rewrite_rules();
-        });
+        // Flush rewrite rules when relevant settings change
+        add_action('update_option_woonoow_appearance_settings', function($old_value, $new_value) {
+            $old_general = $old_value['general'] ?? [];
+            $new_general = $new_value['general'] ?? [];
+            
+            // Only flush if spa_mode, spa_page, or use_browser_router changed
+            $needs_flush = 
+                ($old_general['spa_mode'] ?? '') !== ($new_general['spa_mode'] ?? '') ||
+                ($old_general['spa_page'] ?? '') !== ($new_general['spa_page'] ?? '') ||
+                ($old_general['use_browser_router'] ?? true) !== ($new_general['use_browser_router'] ?? true);
+            
+            if ($needs_flush) {
+                flush_rewrite_rules();
+            }
+        }, 10, 2);
         
         // Redirect WooCommerce pages to SPA routes early (before template loads)
         add_action('template_redirect', [__CLASS__, 'redirect_wc_pages_to_spa'], 5);