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/component/month-selector.test.ts

@@ -0,0 +1,37 @@
+import { describe, expect, it } from 'vitest';
+import MonthSelector, {
+  formatMonth,
+  generateMonthOptions
+} from '../../src/lib/components/layout/MonthSelector.svelte';
+import { selectedPeriod, setPeriod } from '../../src/lib/stores/period';
+
+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('MonthSelector component', () => {
+  it('exports a component module', () => {
+    expect(MonthSelector).toBeDefined();
+  });
+
+  it('formats month labels', () => {
+    expect(formatMonth('2026-02')).toBe('Feb 2026');
+  });
+
+  it('builds +/- 6 month options', () => {
+    const options = generateMonthOptions(new Date(2026, 1, 1));
+    expect(options).toHaveLength(13);
+    expect(options[0]).toBe('2025-08');
+    expect(options[12]).toBe('2026-08');
+  });
+
+  it('selection updates period store', () => {
+    setPeriod('2026-02');
+    expect(getStoreValue(selectedPeriod)).toBe('2026-02');
+  });
+});