fix(header_bar): increase compact height

Increases Compact header height to 40 (from 36), to better accommodate larger widgets (e.g. the search bar). This makes it distinct from the SSD header, which is now addressed by the `_` match case.
Also makes the horizontal header padding fixed at 8, so that the app window padding can also be 8.

cosmic-theme/src/model/density.rs

@@ -1,10 +1,14 @@
 use crate::Spacing;
 use serde::{Deserialize, Serialize};
 
+/// Density options for the Cosmic theme
 #[derive(Copy, Clone, Debug, Default, Eq, PartialEq, Serialize, Deserialize)]
 pub enum Density {
+    /// Lower padding/spacing of elements
     Compact,
+    /// Higher padding/spacing of elements
     Spacious,
+    /// Standard padding/spacing of elements
     #[default]
     Standard,
 }