Fix dock icon not auto-switching with system dark mode (#1928)

* Fix dock icon not auto-switching with system dark mode

The automatic icon mode relied on the asset catalog to handle
appearance-based icon selection, but the compiled Assets.car does not
include dark variant renditions for AppIcon. This meant setting
applicationIconImage to nil had no effect — the icon stayed on the
light variant regardless of system appearance.

Replace the nil-reset approach with an active KVO observer on
NSApp.effectiveAppearance that programmatically swaps between
AppIconDark and AppIconLight images when in automatic mode.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

* Address review feedback: fix async race and harden singleton

- Add guard in async callback to skip stale updates after stopObserving()
- Add private init() to prevent external instantiation of singleton
- Remove unused .new KVO option

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: CHE-3 <schumannzheng@gmail.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Sources/cmuxApp.swift

@@ -3695,14 +3695,14 @@ enum AppIconSettings {
     static func applyIcon(_ mode: AppIconMode) {
         switch mode {
         case .automatic:
-            // Let the asset catalog handle appearance-based icon selection (macOS 15+).
-            // Reset to the default bundle icon.
-            NSApplication.shared.applicationIconImage = nil
+            AppIconAppearanceObserver.shared.startObserving()
         case .light:
+            AppIconAppearanceObserver.shared.stopObserving()
             if let icon = NSImage(named: "AppIconLight") {
                 NSApplication.shared.applicationIconImage = icon
             }
         case .dark:
+            AppIconAppearanceObserver.shared.stopObserving()
             if let icon = NSImage(named: "AppIconDark") {
                 NSApplication.shared.applicationIconImage = icon
             }
@@ -3710,6 +3710,37 @@ enum AppIconSettings {
     }
 }
 
+final class AppIconAppearanceObserver: NSObject {
+    static let shared = AppIconAppearanceObserver()
+    private var observation: NSKeyValueObservation?
+
+    private override init() { super.init() }
+
+    func startObserving() {
+        applyIconForCurrentAppearance()
+        guard observation == nil else { return }
+        observation = NSApp.observe(\.effectiveAppearance, options: []) { [weak self] _, _ in
+            DispatchQueue.main.async {
+                guard let self, self.observation != nil else { return }
+                self.applyIconForCurrentAppearance()
+            }
+        }
+    }
+
+    func stopObserving() {
+        observation?.invalidate()
+        observation = nil
+    }
+
+    private func applyIconForCurrentAppearance() {
+        let isDark = NSApp.effectiveAppearance.bestMatch(from: [.darkAqua, .aqua]) == .darkAqua
+        let imageName = isDark ? "AppIconDark" : "AppIconLight"
+        if let icon = NSImage(named: imageName) {
+            NSApplication.shared.applicationIconImage = icon
+        }
+    }
+}
+
 enum QuitWarningSettings {
     static let warnBeforeQuitKey = "warnBeforeQuitShortcut"
     static let defaultWarnBeforeQuit = true