iOS: keep the screen on while phrasedeck is in front

ios/Phrasedeck/KeepAwake.swift

@@ -0,0 +1,6 @@
+import SwiftUI
+
+/// Keeps the screen on while the app is in front, so the deck is ready to tap (#159).
+enum KeepAwake {
+    static func idleTimerDisabled(for phase: ScenePhase) -> Bool { phase == .active }
+}

ios/Phrasedeck/PhrasedeckApp.swift

@@ -10,6 +10,7 @@ struct PhrasedeckApp: App {
     private let stub: StubHelper?
     /// Bonjour discovery (#11); nil under UI tests, so no local-network prompt.
     @State private var browser: NearbyBrowser?
+    @Environment(\.scenePhase) private var scenePhase
 
     init() {
         #if DEBUG
@@ -55,6 +56,9 @@ struct PhrasedeckApp: App {
                     browser?.start()
                 }
         }
+        .onChange(of: scenePhase, initial: true) { _, phase in
+            UIApplication.shared.isIdleTimerDisabled = KeepAwake.idleTimerDisabled(for: phase)
+        }
         .modelContainer(container)
         // UI tests keep the button size in their throwaway defaults.
         .defaultAppStorage(stub?.defaults ?? .standard)

ios/PhrasedeckTests/KeepAwakeTests.swift

@@ -0,0 +1,15 @@
+import SwiftUI
+import Testing
+@testable import Phrasedeck
+
+/// The screen stays on only while the app is in front (#159).
+struct KeepAwakeTests {
+    @Test(arguments: [
+        (ScenePhase.active, true),
+        (.inactive, false),
+        (.background, false),
+    ])
+    func idleTimerIsOffOnlyWhileActive(phase: ScenePhase, disabled: Bool) {
+        #expect(KeepAwake.idleTimerDisabled(for: phase) == disabled)
+    }
+}