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/App.tsx

@@ -954,6 +954,7 @@ const CADEditor: React.FC<CADEditorProps> = ({ projectId, token, onNavigateBack
           selectedTemplate={selectedTemplate}
           onTemplateSelect={handleTemplateSelect}
           className={mobileLeftOpen ? 'open' : ''}
+          onCollapse={() => setMobileLeftOpen(false)}
         />
         <CanvasArea
           cursorPos={cursorPos}
@@ -1017,6 +1018,7 @@ const CADEditor: React.FC<CADEditorProps> = ({ projectId, token, onNavigateBack
           onKISuggestionClick={handleSuggestionClick}
           kiLoading={kiLoading}
           className={mobileRightOpen ? 'open' : ''}
+          onCollapse={() => setMobileRightOpen(false)}
         />
       </div>
       <CommandLine