✨ feat: Add skeleton loading states for sidebar navigation

Add comprehensive skeleton screen implementation for sidebar to improve loading UX, matching the existing headerbar skeleton pattern.

## Features Added
- **Sidebar skeleton screens**: Complete 1:1 recreation of sidebar structure during loading
- **Responsive skeleton layouts**: Different layouts for expanded (164×30px) and collapsed (44×44px) states
- **Skeleton component enhancements**: Extended SkeletonWrapper with new skeleton types (sidebar, button, sidebarNavItem, sidebarGroupTitle)
- **Minimum loading time**: Integrated useMinimumLoadingTime hook with 500ms duration for smooth UX

## Layout Specifications
- **Expanded nav items**: 164×30px with 8px horizontal margins and 3px vertical margins
- **Collapsed nav items**: 44×44px with 4px bottom margin and 8px horizontal margins
- **Collapse button**: 156×24px (expanded) / 36×24px (collapsed) with rounded corners
- **Container padding**: 12px top padding, 8px horizontal margins
- **Group labels**: 4px 15px 8px padding matching real sidebar-group-label styles

## Code Improvements
- **Refactored skeleton rendering**: Eliminated code duplication using reusable components (NavRow, CollapsedRow)
- **Configuration-driven sections**: Sections defined as config objects with title widths and item widths
- **Fixed width calculations**: Removed random width generation, using precise fixed widths per menu item
- **Proper CSS class alignment**: Uses real sidebar CSS classes (sidebar-section, sidebar-group-label, sidebar-divider)

## UI/UX Enhancements
- **Bottom-aligned collapse button**: Fixed positioning using margin-top: auto to stay at viewport bottom
- **Accurate spacing**: Matches real sidebar margins, padding, and spacing exactly
- **Skeleton stability**: Fixed width values prevent layout shifts during loading
- **Clean file structure**: Removed redundant HeaderBar.js export file

## Technical Details
- Extended SkeletonWrapper component with sidebar-specific skeleton types
- Integrated skeleton loading state management in SiderBar component
- Added support for collapsed state awareness in skeleton rendering
- Implemented precise dimension matching for pixel-perfect loading states

Closes: Sidebar skeleton loading implementation

web/src/components/layout/PageLayout.jsx

@@ -17,7 +17,7 @@ along with this program. If not, see <https://www.gnu.org/licenses/>.
 For commercial licensing, please contact support@quantumnous.com
 */
 
-import HeaderBar from './HeaderBar';
+import HeaderBar from './headerbar';
 import { Layout } from '@douyinfe/semi-ui';
 import SiderBar from './SiderBar';
 import App from '../../App';