test(e2e): add GA4 tracking, mobile, user journey tests and update visual baselines

- Add GA4 event tracking E2E tests (4 cases: page view, form submit, button click, product page)
- Add mobile-specific E2E tests (16 cases: navigation, form, product browsing)
- Add user journey tests (UJ-01: homepage to contact, UJ-02: product discovery)
- Fix E2E test selectors and assertions for consistency
- Update visual regression baselines across all browsers (chromium/firefox/webkit)
- Update Playwright config for new test files

e2e/cases-filter.spec.ts

@@ -1,6 +1,6 @@
 import { test, expect } from '@playwright/test';
 
-test('案例页行业筛选按钮可以正确过滤列表', async ({ page }) => {
+test('案例页行业筛选按钮可以正确过滤列表', { tag: '@regression' }, async ({ page }) => {
   await page.goto('/cases', { waitUntil: 'domcontentloaded' });
 
   // 默认显示全部 6 个案例