Theme · Platform: one platform for buttons and caps, icon themes, frosted panes

theme::Platform moves out of typography and shell's ControlsStyle folds
into it; the theme's platform starts as the system's. Window buttons Ely
draws, the title bar's layout and every key cap follow it, secondary
included, through typography::keys::keystroke; a title bar whose buttons
the OS draws follows the OS. primitives::IconTheme lays an owner's rules
by file name and extension over Ely's map, for every file icon.
shell::Vibrancy fills a pane with the palette's new glass, translucent
so a window opened Blurred shows its backdrop, solid under high contrast.
gpui 0.2.2 has no Mica; that line stays blocked.

src/theme/mod.rs

@@ -9,6 +9,7 @@ mod media;
 mod messaging;
 mod onboarding;
 mod palette;
+mod platform;
 mod project;
 mod syntax;
 mod tokens;
@@ -32,6 +33,7 @@ pub use media::MediaSizes;
 pub use messaging::MessagingSizes;
 pub use onboarding::OnboardingSizes;
 pub use palette::{HUE_NAMES, Mix, Palette, Syntax};
+pub use platform::Platform;
 pub use project::ProjectSizes;
 pub use syntax::{SyntaxTheme, syntax_themes};
 pub use tokens::{
@@ -69,6 +71,7 @@ pub struct Theme {
     pub font_family: SharedString,
     pub mono_family: SharedString,
     pub reduced_motion: bool,
+    pub platform: Platform,
 }
 
 impl Global for Theme {}
@@ -97,6 +100,7 @@ impl Theme {
             font_family: "Inter".into(),
             mono_family: "JetBrains Mono".into(),
             reduced_motion: false,
+            platform: Platform::current(),
         });
     }