Add a DEBUG screen-marker hook so Simulator taps can be verified

`-qaScreenMarker` paints the screen flat magenta, which lets a macOS capture
locate the device-screen rectangle exactly and map simulator-screenshot pixels
to click coordinates. `simctl` cannot drive taps and the project has no UI-test
target, so this is the only way to test real touch behaviour on the sheet.
Guarded by #if DEBUG, exactly like the existing -qaCopyPostcode hook.

FuelBoard/FuelBoardApp.swift

@@ -21,7 +21,7 @@ struct FuelBoardApp: App {
 
     var body: some Scene {
         WindowGroup {
-            ContentView()
+            rootView
                 .onOpenURL(perform: handleOpenURL)
                 .onReceive(NotificationCenter.default.publisher(for: .fuelBoardWatchRefreshRequested)) { _ in
                     WatchSyncManager.shared.pushSnapshot()
@@ -32,6 +32,24 @@ struct FuelBoardApp: App {
         }
     }
 
+    /// QA hook (Debug builds only): `-qaScreenMarker` paints the whole screen a
+    /// flat magenta. A macOS capture of the Simulator window can then locate the
+    /// device-screen rectangle exactly, which yields a pixel-accurate mapping
+    /// from simulator screenshots to click coordinates. That is the only way to
+    /// drive REAL taps into the Simulator: `simctl` cannot, and this project has
+    /// no UI-test target. Everything else renders ContentView as normal.
+    @ViewBuilder private var rootView: some View {
+        #if DEBUG
+        if ProcessInfo.processInfo.arguments.contains("-qaScreenMarker") {
+            Color(red: 1, green: 0, blue: 1).ignoresSafeArea()
+        } else {
+            ContentView()
+        }
+        #else
+        ContentView()
+        #endif
+    }
+
     #if DEBUG
     private func startQALiveActivity(fuel: FuelType) {
         let state = FuelBoardLiveActivityAttributes.ContentState(