App applies conditional state, supports theme setting

The prior light/dark mode awareness work works on surface-level APIs. As
a result, configurations used at the app-level (such as split divider
colors, inactive split opacity, etc.) are not aware of the current theme
configurations and default to the "light" theme.

This commit adds APIs to specify app-level color scheme changes. This
changes the configuration for the app and sets the default conditional
state to use that new theme. This latter point makes it so that future
surfaces use the correct theme on load rather than requiring some apprt
event loop ticks. Some users have already reported a short "flicker" to
load the correct theme, so this should help alleviate that.

src/apprt/embedded.zig

@@ -1357,6 +1357,22 @@ pub const CAPI = struct {
         return v.hasGlobalKeybinds();
     }
 
+    /// Update the color scheme of the app.
+    export fn ghostty_app_set_color_scheme(v: *App, scheme_raw: c_int) void {
+        const scheme = std.meta.intToEnum(apprt.ColorScheme, scheme_raw) catch {
+            log.warn(
+                "invalid color scheme to ghostty_surface_set_color_scheme value={}",
+                .{scheme_raw},
+            );
+            return;
+        };
+
+        v.core_app.colorSchemeEvent(v, scheme) catch |err| {
+            log.err("error setting color scheme err={}", .{err});
+            return;
+        };
+    }
+
     /// Returns initial surface options.
     export fn ghostty_surface_config_new() apprt.Surface.Options {
         return .{};