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/e2e/theme.spec.ts

@@ -0,0 +1,37 @@
+import { expect, test } from '@playwright/test';
+
+async function expectTheme(page: import('@playwright/test').Page, theme: 'light' | 'dark') {
+  await expect
+    .poll(async () =>
+      page.evaluate(() => ({
+        attr: document.documentElement.getAttribute('data-theme'),
+        stored: localStorage.getItem('headroom_theme')
+      }))
+    )
+    .toEqual({ attr: theme, stored: theme });
+
+}
+
+test.describe('Theme persistence', () => {
+  test('applies dark theme from localStorage on reload', async ({ page }) => {
+    await page.goto('/login');
+
+    await page.evaluate(() => {
+      localStorage.setItem('headroom_theme', 'dark');
+    });
+
+    await page.reload();
+    await expectTheme(page, 'dark');
+  });
+
+  test('applies light theme from localStorage on reload', async ({ page }) => {
+    await page.goto('/login');
+
+    await page.evaluate(() => {
+      localStorage.setItem('headroom_theme', 'light');
+    });
+
+    await page.reload();
+    await expectTheme(page, 'light');
+  });
+});