macOS: fix icon style not updating properly on Tahoe (#12180)

This is a regression from #9983. When resetting to default, we shouldn't
use the representation of the icon, which will prevent the icon from
updating after system settings change.

1. Delete `macos-icon` config if it exists and reload.
2. Go to **System Settings -> Appearance** and change **Icon & widget
style** to any one other than Default, and observe the app icon.


<img width="228" height="179" alt="image"
src="https://github.com/user-attachments/assets/e53274f8-b679-4d6f-8e0b-edfd7d17811d"
/>

> A temporary workaround to this issue is to reload the config.

This pr resets the `NSDockTile.contentView`, which will let AppKit
revert back to `Ghostty.icon`.



https://github.com/user-attachments/assets/06ab0519-225b-45e1-85a5-a22832a36177

macos/Sources/Features/Custom App Icon/DockTilePlugin.swift

@@ -82,7 +82,7 @@ class DockTilePlugin: NSObject, NSDockTilePlugIn {
     /// Reset the application icon and dock tile icon to the default.
     private func resetIcon(dockTile: NSDockTile) {
         let appBundlePath = self.ghosttyAppURL?.path
-        let appIcon: NSImage
+        let appIcon: NSImage?
         if #available(macOS 26.0, *) {
             // Reset to the default (glassy) icon.
             if let appBundlePath {
@@ -93,21 +93,8 @@ class DockTilePlugin: NSObject, NSDockTilePlugIn {
             // Use the `Blueprint` icon to distinguish Debug from Release builds.
             appIcon = pluginBundle.image(forResource: "BlueprintImage")!
             #else
-            // Get the composed icon from the app bundle.
-            if let appBundlePath,
-                let iconRep = NSWorkspace.shared.icon(forFile: appBundlePath)
-                .bestRepresentation(
-                    for: CGRect(origin: .zero, size: dockTile.size),
-                    context: nil,
-                    hints: nil
-            ) {
-                appIcon = NSImage(size: dockTile.size)
-                appIcon.addRepresentation(iconRep)
-            } else {
-                // If something unexpected happens on macOS 26,
-                // fall back to a bundled icon.
-                appIcon = pluginBundle.image(forResource: "AppIconImage")!
-            }
+            // Reset to Ghostty.icon
+            appIcon = nil
             #endif
         } else {
             // Use the bundled icon to keep the corner radius consistent with pre-Tahoe apps.
@@ -126,9 +113,14 @@ class DockTilePlugin: NSObject, NSDockTilePlugIn {
 }
 
 private extension NSDockTile {
-    func setIcon(_ newIcon: NSImage) {
+    func setIcon(_ newIcon: NSImage?) {
         // Update the Dock tile on the main thread.
         DispatchQueue.main.async {
+            guard let newIcon else {
+                self.contentView = nil
+                self.display()
+                return
+            }
             let iconView = NSImageView(frame: CGRect(origin: .zero, size: self.size))
             iconView.wantsLayer = true
             iconView.image = newIcon