ci(frontend): shim storage in test setup

Add a Vitest setup storage shim for localStorage and sessionStorage when Node/jsdom does not provide usable storage.

This fixes the Node 26 frontend CI failures where tests accessed localStorage during setup.

frontend/src/__tests__/setup.ts

@@ -9,6 +9,67 @@ class MockResizeObserver {
 }
 globalThis.ResizeObserver = globalThis.ResizeObserver ?? MockResizeObserver;
 
+type StorageName = 'localStorage' | 'sessionStorage';
+
+class TestStorage {
+  private readonly store = new Map<string, string>();
+
+  get length() {
+    return this.store.size;
+  }
+
+  clear() {
+    this.store.clear();
+  }
+
+  getItem(key: string) {
+    return this.store.get(String(key)) ?? null;
+  }
+
+  key(index: number) {
+    return Array.from(this.store.keys())[index] ?? null;
+  }
+
+  removeItem(key: string) {
+    this.store.delete(String(key));
+  }
+
+  setItem(key: string, value: string) {
+    this.store.set(String(key), String(value));
+  }
+}
+
+function getUsableStorage(name: StorageName): Storage {
+  try {
+    const storage = window[name];
+    storage.setItem('__sencho_storage_probe__', '1');
+    storage.removeItem('__sencho_storage_probe__');
+    return storage;
+  } catch {
+    return new TestStorage() as Storage;
+  }
+}
+
+function defineStorage(name: StorageName, storage: Storage) {
+  for (const target of [window, globalThis]) {
+    try {
+      Object.defineProperty(target, name, {
+        value: storage,
+        configurable: true,
+        enumerable: true,
+        writable: true,
+      });
+    } catch {
+      Reflect.set(target, name, storage);
+    }
+  }
+}
+
+if (typeof window !== 'undefined') {
+  defineStorage('localStorage', getUsableStorage('localStorage'));
+  defineStorage('sessionStorage', getUsableStorage('sessionStorage'));
+}
+
 if (typeof window !== 'undefined' && !window.matchMedia) {
   window.matchMedia = vi.fn().mockImplementation((query: string) => ({
     matches: false,