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/settings.spec.ts

@@ -9,9 +9,7 @@ test.describe("Settings", () => {
     })
 
     test("settings dialog opens", async ({ page }) => {
-        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()
 
@@ -20,9 +18,7 @@ test.describe("Settings", () => {
     })
 
     test("language selection is available", async ({ page }) => {
-        const settingsButton = page.locator(
-            'button[aria-label*="settings"], button:has(svg[class*="settings"])',
-        )
+        const settingsButton = page.locator('[data-testid="settings-button"]')
         await settingsButton.click()
 
         const dialog = page.locator('[role="dialog"]')
@@ -33,9 +29,7 @@ test.describe("Settings", () => {
     })
 
     test("draw.io theme toggle exists", async ({ page }) => {
-        const settingsButton = page.locator(
-            'button[aria-label*="settings"], button:has(svg[class*="settings"])',
-        )
+        const settingsButton = page.locator('[data-testid="settings-button"]')
         await settingsButton.click()
 
         const dialog = page.locator('[role="dialog"]')