Implement live collection preview system with contextual template selection

Replace isolated template previews with live collection reconstruction:
- Frontend now reconstructs collection container with all template variants
- Users click directly on rendered templates in proper CSS context
- Perfect preservation of grid/flex layouts and responsive behavior
- Simplified API: preview endpoint returns container_html + templates for frontend reconstruction
- Enhanced UX: WYSIWYG template selection shows exactly what will be added
- Removed redundant templates endpoint in favor of unified preview approach

Backend changes:
- Add GET /api/collections/{id}/preview endpoint
- Remove GET /api/collections/{id}/templates endpoint
- Return container HTML + templates for frontend reconstruction

Frontend changes:
- Replace isolated template modal with live collection preview
- Add generateLivePreview() method for container reconstruction
- Update CollectionManager to use preview API
- Add interactive CSS styling for template selection

This provides true contextual template selection where CSS inheritance,
grid layouts, and responsive design work perfectly in preview mode.

lib/src/core/api-client.js

@@ -243,25 +243,25 @@ export class ApiClient {
     }
 
     /**
-     * Get available templates for a collection
+     * Get collection preview data (container + templates for frontend reconstruction)
      * @param {string} collectionId - Collection ID
-     * @returns {Promise<Array>} Array of collection templates
+     * @returns {Promise<Object>} Object with collection_id, container_html, and templates
      */
-    async getCollectionTemplates(collectionId) {
+    async getCollectionPreview(collectionId) {
         try {
             const collectionsUrl = this.getCollectionsUrl();
-            const response = await fetch(`${collectionsUrl}/${collectionId}/templates?site_id=${this.siteId}`);
+            const response = await fetch(`${collectionsUrl}/${collectionId}/preview?site_id=${this.siteId}`);
             
             if (response.ok) {
                 const result = await response.json();
-                return result.templates || [];
+                return result;
             } else {
-                console.warn(`⚠️ Failed to fetch collection templates (${response.status}): ${collectionId}`);
-                return [];
+                console.warn(`⚠️ Failed to fetch collection preview (${response.status}): ${collectionId}`);
+                return null;
             }
         } catch (error) {
-            console.error('Failed to fetch collection templates:', collectionId, error);
-            return [];
+            console.error('Failed to fetch collection preview:', collectionId, error);
+            return null;
         }
     }