Feat/sidebar board tree

[Surya10102000]

The PRs closes issue #202
This adds a new collapsible board list component to the sidebar navigation, improving the user experience by organizing boards in an expandable/collapsible tree structure.

after.mp4

The component fully supports both light and dark themes and preserves the sidebar collapse state.

Changes

• New CollapsibleBoardList component with expandable/collapsible board tree
• Loading state with spinner while fetching boards
• Scrollable board list with custom scrollbar styling
• Responsive behavior for mobile devices

Modified

• SideNavigation.tsx - Integrated the new collapsible board list component
• Added proper typing for board data structures

Testing Done

• Tested board list expansion/collapse in both light and dark themes
• Verified keyboard shortcuts work correctly
• Tested responsive behavior on mobile viewports
• Confirmed sidebar state persists after page refresh
• Verified loading states display properly
• Tested navigation to individual boards
• Verified current board highlighting works
• Tested scroll behavior with many boards

Light Theme