✨ 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/index.css

@@ -184,6 +184,7 @@ code {
   justify-content: center;
   align-items: center;
   padding: 12px;
+  margin-top: auto;
   cursor: pointer;
   background-color: var(--semi-color-bg-0);
   position: sticky;