refactor: simplify KanbanFilters and SourceQuickFilter components

- Removed unused imports and state management for dropdowns, enhancing performance and readability.
- Replaced custom dropdown implementation with a reusable `Dropdown` component for better consistency across the UI.
- Updated button styles and logic for clearer user interaction in the filters.
- Integrated dropdowns into the `SourceQuickFilter` for improved functionality and user experience.

src/components/ui-showcase/sections/index.ts

@@ -5,3 +5,4 @@ export { FormsSection } from './FormsSection';
 export { NavigationSection } from './NavigationSection';
 export { FeedbackSection } from './FeedbackSection';
 export { DataDisplaySection } from './DataDisplaySection';
+export { DropdownsSection } from './DropdownsSection';