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/components/RightSidebar.tsx

@@ -23,9 +23,13 @@ const RightSidebar: React.FC<RightSidebarProps> = ({
   onRenameBlock, onDuplicateBlock, onDeleteBlock, onSvgImport, onSaveGroupAsBlock,
   onBlockCategoryChange, onBlockSearch, onDragBlock,
   kiMessages, kiSuggestions, onKISend, onKISuggestionClick, kiLoading, onUpdateElement,
+  onCollapse,
 }) => {
   return (
     <aside className={`rightbar${className ? ' ' + className : ''}`} aria-label="Eigenschaften-Panels">
+      <button className="rightbar-close" aria-label="Rechte Leiste schließen" onClick={onCollapse}>
+        <svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"><polyline points="9 18 15 12 9 6"/></svg>
+      </button>
       <div className="rightbar-tabs" role="tablist">
         {tabs.map((tab) => (
           <button