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.

macos/Sources/App/macOS/AppDelegate.swift

@@ -95,6 +95,9 @@ class AppDelegate: NSObject,
     /// makes our logic very easy.
     private var isVisible: Bool = true
 
+    /// The observer for the app appearance.
+    private var appearanceObserver: NSKeyValueObservation? = nil
+
     override init() {
         terminalManager = TerminalManager(ghostty)
         updaterController = SPUStandardUpdaterController(
@@ -187,6 +190,23 @@ class AppDelegate: NSObject,
             )
         ])
         center.delegate = self
+
+        // Observe our appearance so we can report the correct value to libghostty.
+        self.appearanceObserver = NSApplication.shared.observe(
+            \.effectiveAppearance,
+             options: [.new, .initial]
+        ) { _, change in
+            guard let appearance = change.newValue else { return }
+            guard let app = self.ghostty.app else { return }
+            let scheme: ghostty_color_scheme_e
+            if (appearance.isDark) {
+                scheme = GHOSTTY_COLOR_SCHEME_DARK
+            } else {
+                scheme = GHOSTTY_COLOR_SCHEME_LIGHT
+            }
+
+            ghostty_app_set_color_scheme(app, scheme)
+        }
     }
 
     func applicationDidBecomeActive(_ notification: Notification) {