feat: delete function, sidebar collapse, layer drag&drop, library tree

- Delete: trash button in PropertiesPanel + existing Delete key support
- Left sidebar: desktop collapse toggle button
- Right sidebar: desktop collapse toggle + SVG icon size fix (replaced emoji)
- Layer panel: drag&drop enabled (reorder, move to layers, sub-layers)
- Library tree: BlockLibrary added to left sidebar
- App.tsx: handleReorderLayer, handleAddSubLayer, handleUpdateElement
- styles.css: collapse + drag&drop visual indicators

frontend/src/types/ui.types.ts

@@ -87,6 +87,18 @@ export interface LeftSidebarProps {
   onTemplateSelect?: (templateName: string | null) => void;
   onCollapse?: () => void;
   className?: string;
+  collapsed?: boolean;
+  onToggleCollapse?: () => void;
+  blocks?: BlockDefinition[];
+  onDragBlock?: (blockId: string) => void;
+  onBlockDrop?: (blockId: string, x: number, y: number) => void;
+  onBlockCategoryChange?: (cat: string) => void;
+  onBlockSearch?: (query: string) => void;
+  onRenameBlock?: (id: string, name: string) => void;
+  onDuplicateBlock?: (id: string) => void;
+  onDeleteBlock?: (id: string) => void;
+  onSvgImport?: (svg: string, name: string, category: string) => void;
+  onSaveGroupAsBlock?: (name: string) => void;
 }
 
 export interface CanvasAreaProps {
@@ -160,12 +172,15 @@ export interface RightSidebarProps {
   onUpdateElement?: (el: CADElement) => void;
   className?: string;
   onCollapse?: () => void;
+  collapsed?: boolean;
+  onToggleCollapse?: () => void;
 }
 
 export interface PropertiesPanelProps {
   selectedElement: CADElement | null;
   layers: CADLayer[];
   onUpdateProperty: (key: string, value: unknown) => void;
+  onDelete?: () => void;
 }
 
 export interface LayerPanelProps {