feat(layout): finalize p01 and p02 changes

Complete UI foundation and app layout implementation, stabilize container health checks, and archive both OpenSpec changes after verification.

frontend/tests/unit/period.store.test.ts

@@ -0,0 +1,44 @@
+import { beforeEach, describe, expect, it, vi, type Mock } from 'vitest';
+
+function getStoreValue<T>(store: { subscribe: (run: (value: T) => void) => () => void }): T {
+  let value!: T;
+  const unsubscribe = store.subscribe((current) => {
+    value = current;
+  });
+  unsubscribe();
+  return value;
+}
+
+describe('period store', () => {
+  beforeEach(() => {
+    vi.resetModules();
+    (localStorage.getItem as Mock).mockReturnValue(null);
+  });
+
+  it('initializes with current month', async () => {
+    const store = await import('../../src/lib/stores/period');
+
+    const now = new Date();
+    const expected = `${now.getFullYear()}-${String(now.getMonth() + 1).padStart(2, '0')}`;
+    expect(getStoreValue(store.selectedPeriod)).toBe(expected);
+  });
+
+  it('previousMonth decrements correctly', async () => {
+    const store = await import('../../src/lib/stores/period');
+
+    store.setPeriod('2025-01');
+    store.previousMonth();
+
+    expect(getStoreValue(store.selectedPeriod)).toBe('2024-12');
+    expect(localStorage.setItem).toHaveBeenCalledWith('headroom_selected_period', '2024-12');
+  });
+
+  it('nextMonth increments correctly', async () => {
+    const store = await import('../../src/lib/stores/period');
+
+    store.setPeriod('2025-12');
+    store.nextMonth();
+
+    expect(getStoreValue(store.selectedPeriod)).toBe('2026-01');
+  });
+});