fix: improve test infrastructure based on PR review

- Fix double build in CI: remove redundant build from playwright webServer
- Export chat helpers from shared module for proper unit testing
- Replace waitForTimeout with explicit waits in E2E tests
- Add data-testid attributes to settings and new chat buttons
- Add list reporter for CI to show failures in logs
- Add Playwright browser caching to speed up CI
- Add vitest coverage configuration
- Fix conditional test assertions to use test.skip() instead of silent pass
- Remove unused variables flagged by linter

tests/e2e/smoke.spec.ts

@@ -37,9 +37,7 @@ test.describe("Smoke Tests", () => {
             .waitFor({ state: "visible", timeout: 30000 })
 
         // Click settings button (gear icon)
-        const settingsButton = page.locator(
-            'button[aria-label*="settings"], button:has(svg[class*="settings"])',
-        )
+        const settingsButton = page.locator('[data-testid="settings-button"]')
         await expect(settingsButton).toBeVisible()
         await settingsButton.click()