Fixed sidebar responsiveness

- Created new widget `scrollbar` which can adopt COSMIC styling.
- Increased sidebar icon size by 4 pixels to improve visibility.
- Added padding inside the navigation bar to prevent collision with the scrollbar.
- Increased the max width of the condensed navbar for the scrollbar.

src/widget/mod.rs

@@ -15,3 +15,6 @@ pub use nav::*;
 
 mod toggler;
 pub use toggler::*;
+
+mod scrollbar;
+pub use scrollbar::*;