refactor: address Copilot review on debug overlay

- Lazy-load DebugOverlay via defineAsyncComponent so its chunk (and the
  vueuse perf hooks it pulls in) stays out of the default bundle until the
  developer toggle is enabled.
- Add useDebugMode unit tests mirroring useCrtMode: default-off, persisted
  restore, write-through, and singleton sharing.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/v2/composables/useDebugMode/index.test.ts

@@ -0,0 +1,50 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { nextTick } from "vue";
+
+const KEY = "settings.v2.debugMode";
+
+// `enabled` is a module-level singleton created at import time, so it reads
+// localStorage exactly once on load. Re-import the module fresh after seeding
+// storage to exercise the default / persisted-read paths in isolation.
+async function loadFresh() {
+  vi.resetModules();
+  const { useDebugMode } = await import("./index");
+  return useDebugMode();
+}
+
+beforeEach(() => {
+  localStorage.clear();
+});
+
+afterEach(() => {
+  localStorage.clear();
+});
+
+describe("useDebugMode", () => {
+  it("defaults to off when nothing is persisted", async () => {
+    const { enabled } = await loadFresh();
+    expect(enabled.value).toBe(false);
+  });
+
+  it("restores a previously persisted value on load", async () => {
+    localStorage.setItem(KEY, "true");
+    const { enabled } = await loadFresh();
+    expect(enabled.value).toBe(true);
+  });
+
+  it("persists changes under the 'settings.v2.debugMode' key", async () => {
+    const { enabled } = await loadFresh();
+    enabled.value = true;
+    await nextTick();
+    expect(localStorage.getItem(KEY)).toBe("true");
+  });
+
+  it("shares a single instance across calls within a load", async () => {
+    vi.resetModules();
+    const { useDebugMode } = await import("./index");
+    const a = useDebugMode();
+    const b = useDebugMode();
+    a.enabled.value = true;
+    expect(b.enabled.value).toBe(true);
+  });
+});

frontend/src/v2/layouts/AppLayout.vue

@@ -9,7 +9,13 @@
 // Per-ROM action menus are not app-wide: each GameCard owns its own
 // `MoreMenu` dropdown on the three-dots button. Right-click is left to
 // the browser so "Open in new tab" etc. keep working.
-import { onBeforeUnmount, onMounted, provide, ref } from "vue";
+import {
+  defineAsyncComponent,
+  onBeforeUnmount,
+  onMounted,
+  provide,
+  ref,
+} from "vue";
 import { useRouter } from "vue-router";
 import storeCollections from "@/stores/collections";
 import storePlatforms from "@/stores/platforms";
@@ -17,7 +23,6 @@ import AppNav from "@/v2/components/AppShell/AppNav.vue";
 import BackgroundArt from "@/v2/components/AppShell/BackgroundArt.vue";
 import BottomNav from "@/v2/components/AppShell/BottomNav.vue";
 import CrtOverlay from "@/v2/components/AppShell/CrtOverlay.vue";
-import DebugOverlay from "@/v2/components/AppShell/DebugOverlay.vue";
 import GlobalDialogs from "@/v2/components/Dialogs/GlobalDialogs.vue";
 import SoundtrackMiniPlayer from "@/v2/components/Soundtrack/MiniPlayer.vue";
 import { BACKGROUND_ART_KEY } from "@/v2/composables/useBackgroundArt";
@@ -46,7 +51,12 @@ const collectionsStore = storeCollections();
 const platformsStore = storePlatforms();
 
 // Developer debug overlay — opt-in via Settings → Developer (per-device).
+// Lazily loaded so its chunk (and the vueuse perf hooks it pulls in) is only
+// fetched once the toggle is on, keeping it out of the default bundle.
 const { enabled: debugEnabled } = useDebugMode();
+const DebugOverlay = defineAsyncComponent(
+  () => import("@/v2/components/AppShell/DebugOverlay.vue"),
+);
 
 // Shared reactive background art — views paint covers via the injected setter.
 const layerA = ref<string | null>(null);