macos: do not load the config errors window when there are no errors

Measured on macOS (Apple Silicon) during app launch, via a startup
timeline instrumented across the Swift app and libghostty:

  config apply, errors step:      35.5ms -> 0.1ms
  main() -> first frame rendered: ~126ms -> ~93ms
  main() -> window visible:       ~193ms -> ~173ms

macos/Sources/Features/Settings/ConfigurationErrorsController.swift

@@ -13,7 +13,15 @@ class ConfigurationErrorsController: NSWindowController, NSWindowDelegate, Confi
     @Published var errors: [String] = [] {
         didSet {
             if errors.count == 0 {
-                self.window?.performClose(nil)
+                // Only close the window if it was ever loaded: accessing
+                // `window` on an NSWindowController loads the nib (and our
+                // SwiftUI content view), which takes tens of milliseconds.
+                // This happens on every app launch via the initial config
+                // apply, when there are usually no errors and the window
+                // was never loaded.
+                if isWindowLoaded {
+                    self.window?.performClose(nil)
+                }
             }
         }
     }