test(capacity): Add E2E test for availability save

Add comprehensive E2E test for capacity calendar functionality:
- Login and navigate to capacity page
- Create test team member
- Select team member and wait for calendar
- Change availability from Full day to Half day
- Verify save completes without errors
- Verify change persists via API

Test Results:
- Backend: 76 passed ✅
- Frontend Unit: 10 passed ✅
- E2E: 132 passed, 24 skipped ✅
- New test: 2 passed (chromium + firefox) ✅

Refs: openspec/changes/headroom-foundation

frontend/tests/e2e/capacity-calendar.spec.ts

@@ -0,0 +1,97 @@
+import { test, expect, type Page } from '@playwright/test';
+
+const API_BASE = 'http://localhost:3000';
+
+async function login(page: Page) {
+  await page.goto('/login');
+  await page.fill('input[type="email"]', 'superuser@headroom.test');
+  await page.fill('input[type="password"]', 'password');
+  await page.click('button[type="submit"]');
+  await page.waitForURL('/dashboard');
+}
+
+async function createTeamMember(page: Page, token: string) {
+  const response = await page.request.post(`${API_BASE}/api/team-members`, {
+    headers: {
+      Authorization: `Bearer ${token}`,
+      'Content-Type': 'application/json'
+    },
+    data: {
+      name: `Capacity Calendar Tester ${Date.now()}`,
+      role_id: 1,
+      hourly_rate: 150,
+      active: true
+    }
+  });
+
+  const body = await response.json();
+  return body.id as string;
+}
+
+test.describe('Capacity Calendar', () => {
+  let authToken: string;
+  let memberId: string | null = null;
+
+  test.beforeEach(async ({ page }) => {
+    await login(page);
+    const token = (await page.evaluate(() => localStorage.getItem('headroom_access_token'))) ?? '';
+    authToken = token;
+    memberId = await createTeamMember(page, authToken);
+
+    await page.goto('/capacity');
+    await page.waitForURL('/capacity');
+    await page.waitForResponse((response) => response.url().includes('/api/team-members') && response.status() === 200);
+  });
+
+  test.afterEach(async ({ page }) => {
+    if (memberId) {
+      await page.request.delete(`${API_BASE}/api/team-members/${memberId}`, {
+        headers: { Authorization: `Bearer ${authToken}` }
+      }).catch(() => null);
+      memberId = null;
+    }
+  });
+
+  test('should save availability change with success message', async ({ page }) => {
+    await expect(page.locator('select[aria-label="Select team member"]')).toBeVisible({ timeout: 10000 });
+
+    const teamMemberSelect = page.locator('select[aria-label="Select team member"]');
+    if (memberId) {
+      await teamMemberSelect.selectOption({ value: memberId });
+    }
+
+    await expect(page.locator('[data-testid="capacity-calendar"]')).toBeVisible({ timeout: 10000 });
+
+    const availabilitySelects = page
+      .locator('select[aria-label^="Availability for"]')
+      .filter({ has: page.locator('option[value="1"]') });
+
+    await expect(availabilitySelects.first()).toBeVisible({ timeout: 5000 });
+
+    const firstSelect = availabilitySelects.first();
+    const ariaLabel = await firstSelect.getAttribute('aria-label');
+    const targetDate = ariaLabel?.replace('Availability for ', '') ?? '';
+    expect(targetDate).toBeTruthy();
+
+    await firstSelect.selectOption('0.5');
+
+    await expect(page.locator('text=Saving availability...')).toBeVisible({ timeout: 5000 });
+    await expect(page.locator('text=Saving availability...')).not.toBeVisible({ timeout: 10000 });
+
+    await expect(page.locator('[data-testid="capacity-calendar"] .alert.alert-error')).toHaveCount(0);
+
+    if (memberId) {
+      const period =
+        (await page.evaluate(() => localStorage.getItem('headroom_selected_period'))) ?? '2026-02';
+      const response = await page.request.get(
+        `${API_BASE}/api/capacity?month=${period}&team_member_id=${memberId}`
+      );
+      const body = await response.json();
+      const changedDetail = (body.details as Array<{ date: string; availability: number }>).find(
+        (detail) => detail.date === targetDate
+      );
+      expect(changedDetail).toBeDefined();
+      expect(changedDetail?.availability).toBe(0.5);
+    }
+  });
+});