Implement class-based template differentiation and fix collection item creation

- Add class-based template comparison to differentiate styling variants
- Implement template deduplication based on structure + class signatures
- Add GetCollectionTemplate method to repository interface and implementations
- Fix collection item creation by replacing unimplemented CreateCollectionItemAtomic
- Add template selection modal with auto-default selection in frontend
- Generate meaningful template names from distinctive CSS classes
- Fix unique constraint violations with timestamp-based collection item IDs
- Add collection templates API endpoint for frontend template fetching
- Update simple demo with featured/compact/dark testimonial variants for testing

lib/src/core/api-client.js

@@ -242,6 +242,29 @@ export class ApiClient {
         }
     }
 
+    /**
+     * Get available templates for a collection
+     * @param {string} collectionId - Collection ID
+     * @returns {Promise<Array>} Array of collection templates
+     */
+    async getCollectionTemplates(collectionId) {
+        try {
+            const collectionsUrl = this.getCollectionsUrl();
+            const response = await fetch(`${collectionsUrl}/${collectionId}/templates?site_id=${this.siteId}`);
+            
+            if (response.ok) {
+                const result = await response.json();
+                return result.templates || [];
+            } else {
+                console.warn(`⚠️ Failed to fetch collection templates (${response.status}): ${collectionId}`);
+                return [];
+            }
+        } catch (error) {
+            console.error('Failed to fetch collection templates:', collectionId, error);
+            return [];
+        }
+    }
+
     /**
      * Reorder collection items in bulk
      * @param {string} collectionId - Collection ID