This commit refactors the sidebar component to use a struct for better
organization and maintainability. The `data` and `sindex` fields are now
exported as `Data` and `Sindex` respectively, allowing for easier access
and modification. Additionally, the styling for the selected sidebar
item has been updated for improved visual clarity.
Introduce a new `Sidebar` component to encapsulate sidebar logic and
rendering. This component utilizes Bubble Tea and Lipgloss for UI
elements. The main application now integrates this component for
displaying sidebar content.