feat: state platform metrics and chrome in theme

crates/theme/src/system.rs

@@ -1,6 +1,6 @@
 //! The normalized snapshot a platform adapter produces.
 
-use gpui::{Hsla, Pixels};
+use gpui::{Hsla, Pixels, px};
 
 use crate::ThemeVariant;
 
@@ -43,6 +43,17 @@ impl SourceKind {
     pub fn scrollbar_autohide(self) -> bool {
         matches!(self, Self::Macos | Self::Omarchy | Self::Gnome)
     }
+
+    /// The base spacing unit:
+    ///
+    /// - GNOME lays out on a 6px grid
+    /// - Other platform on 4px grid
+    pub fn spacing_unit(self) -> Pixels {
+        match self {
+            Self::Gnome | Self::Omarchy => px(6.),
+            _ => px(4.),
+        }
+    }
 }
 
 /// A partial palette with optional colors.
@@ -83,8 +94,22 @@ pub struct SystemTheme {
     pub palette: PartialPalette,
     /// The platform's stated control corner radius, if it has one.
     pub control_radius: Option<Pixels>,
+    /// The platform's stated menu/popover radius, if it has one.
+    pub overlay_radius: Option<Pixels>,
+    /// The platform's stated checkbox radius, if it has one.
+    pub sm_radius: Option<Pixels>,
+    /// The platform's stated control height, if it has one.
+    pub control_height: Option<Pixels>,
     /// The platform's stated text-field height, if it has one.
     pub input_height: Option<Pixels>,
+    /// The platform's stated menu row height, if it has one.
+    pub menu_item_height: Option<Pixels>,
+    /// The platform's stated switch track height, if it has one.
+    pub switch_height: Option<Pixels>,
+    /// The platform's stated switch track width, if it has one.
+    pub switch_width: Option<Pixels>,
+    /// The platform's stated switch knob size, if it has one.
+    pub switch_knob: Option<Pixels>,
     /// The platform's native titlebar height, if it states one.
     pub titlebar_height: Option<Pixels>,
     /// The platform's native root font size, if it has one.
@@ -100,7 +125,14 @@ impl SystemTheme {
             accent: None,
             palette: PartialPalette::default(),
             control_radius: None,
+            overlay_radius: None,
+            sm_radius: None,
+            control_height: None,
             input_height: None,
+            menu_item_height: None,
+            switch_height: None,
+            switch_width: None,
+            switch_knob: None,
             titlebar_height: None,
             root_font_size: None,
             accessibility: AccessibilityPreferences::default(),