fix: add close buttons to mobile sidebars

- RightSidebar: added onCollapse prop and close button in header

- LeftSidebar: wired existing onCollapse prop in App.tsx

- CSS: show .leftbar-toggle and .rightbar-close on mobile

- Both buttons call setMobile{Left,Right}Open(false) to close overlays

frontend/src/types/ui.types.ts

@@ -159,6 +159,7 @@ export interface RightSidebarProps {
   kiLoading?: boolean;
   onUpdateElement?: (el: CADElement) => void;
   className?: string;
+  onCollapse?: () => void;
 }
 
 export interface PropertiesPanelProps {