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/db/repository.go

@@ -22,6 +22,7 @@ type ContentRepository interface {
 	CreateCollectionTemplate(ctx context.Context, siteID, collectionID, name, htmlTemplate string, isDefault bool) (*CollectionTemplateItem, error)
 	CreateCollectionItem(ctx context.Context, siteID, collectionID, itemID string, templateID int, htmlContent string, position int, lastEditedBy string) (*CollectionItemWithTemplate, error)
 	CreateCollectionItemAtomic(ctx context.Context, siteID, collectionID string, templateID int, lastEditedBy string) (*CollectionItemWithTemplate, error)
+	GetMaxPosition(ctx context.Context, siteID, collectionID string) (int, error)
 	ReorderCollectionItems(ctx context.Context, siteID, collectionID string, items []CollectionItemPosition, lastEditedBy string) error
 
 	// Transaction support