fix: prevent duplicate window on reopen from dock

Use Window scene instead of WindowGroup to prevent SwiftUI from
creating a second window when the app is reopened via dock icon
while the original window is hidden (not closed).

WindowGroup is designed for multi-window apps and creates a new
window instance when no visible window exists. Window is a single-
instance scene type that correctly handles the hidden window case.

Author: sozercan

App/KasetApp.swift

@@ -112,7 +112,7 @@ struct KasetApp: App {
     }
 
     var body: some Scene {
-        WindowGroup {
+        Window("Kaset", id: "main") {
             // Skip UI during unit tests to prevent window spam
             if UITestConfig.isRunningUnitTests, !UITestConfig.isUITestMode {
                 Color.clear