Improve collection management: fix template selection UI and item positioning

This commit addresses multiple collection management issues to improve user experience:

## Template Selection Modal Improvements
- Replace inline styles with CSS classes for reliable visual feedback
- Fix default template selection conflicts that showed multiple templates as selected
- Add styled template previews that show actual CSS styling differences
- Improve modal responsiveness and visual hierarchy

## Collection Item Creation Fixes
- Fix empty collection items with no content/height that were unclickable
- Preserve template placeholder content during item creation instead of clearing it
- Implement proper positioning system using GetMaxPosition to place new items at collection end
- Add position calculation logic to prevent new items from jumping to beginning

## Backend Positioning System
- Add GetMaxPosition method to all repository implementations (SQLite, PostgreSQL, HTTPClient)
- Update CreateCollectionItemFromTemplate to calculate correct position (maxPos + 1)
- Maintain reconstruction ordering by position ASC for consistent item placement

## Frontend Template Selection
- CSS class-based selection states replace problematic inline style manipulation
- Template previews now render actual HTML with real page styling
- Improved hover states and selection visual feedback
- Fixed auto-selection interference with user interaction

These changes ensure collection items appear in expected order and template selection
provides clear visual feedback with actual styling previews.

internal/engine/collection.go

@@ -246,12 +246,9 @@ func (e *ContentEngine) processChildElementsAsContent(childElement *html.Node, s
 			// Set the data-content-id attribute
 			SetAttribute(n, "data-content-id", contentID)
 
-			// Clear content - this will be hydrated during reconstruction
-			for child := n.FirstChild; child != nil; {
-				next := child.NextSibling
-				n.RemoveChild(child)
-				child = next
-			}
+			// Keep content for initial display - don't clear it
+			// The content is already stored in the database and will be available for editing
+			// Preserving content ensures elements have height and are clickable
 		}
 	})
 
@@ -271,12 +268,8 @@ func (e *ContentEngine) generateStructuralTemplateFromChild(childElement *html.N
 				// Set the data-content-id attribute
 				SetAttribute(n, "data-content-id", contentEntries[entryIndex].ID)
 
-				// Clear content - this will be hydrated during reconstruction
-				for child := n.FirstChild; child != nil; {
-					next := child.NextSibling
-					n.RemoveChild(child)
-					child = next
-				}
+				// Keep content for structural template - ensures elements have height and are clickable
+				// Content is stored separately in database for editing
 
 				entryIndex++
 			}
@@ -352,9 +345,17 @@ func (e *ContentEngine) CreateCollectionItemFromTemplate(
 		return nil, fmt.Errorf("failed to generate structural template: %w", err)
 	}
 
-	// Create collection item with structural template
+	// Get next position to place new item at the end of collection
+	maxPosition, err := e.client.GetMaxPosition(context.Background(), siteID, collectionID)
+	if err != nil {
+		return nil, fmt.Errorf("failed to get max position for collection %s: %w", collectionID, err)
+	}
+	nextPosition := maxPosition + 1
+	fmt.Printf("🔢 Max position for collection %s: %d, assigning new item position: %d\n", collectionID, maxPosition, nextPosition)
+
+	// Create collection item with structural template at end position
 	collectionItem, err := e.client.CreateCollectionItem(context.Background(),
-		siteID, collectionID, itemID, templateID, structuralTemplate, 0, lastEditedBy,
+		siteID, collectionID, itemID, templateID, structuralTemplate, nextPosition, lastEditedBy,
 	)
 	if err != nil {
 		return nil, fmt.Errorf("failed to create collection item: %w", err)