refactor: add shared fixtures and test.step() patterns

- Add tests/e2e/lib/fixtures.ts with shared test helpers
- Add tests/e2e/fixtures/diagrams.ts with XML test data
- Add expectBeforeAndAfterReload() helper for persistence tests
- Add test.step() for better test reporting in complex tests
- Consolidate mock helpers into fixtures module
- Reduce code duplication across 17 test files

tests/e2e/settings.spec.ts

@@ -1,41 +1,33 @@
-import { expect, test } from "@playwright/test"
+import {
+    expect,
+    getIframe,
+    getSettingsButton,
+    openSettings,
+    test,
+} from "./lib/fixtures"
 
 test.describe("Settings", () => {
     test.beforeEach(async ({ page }) => {
         await page.goto("/", { waitUntil: "networkidle" })
-        await page
-            .locator("iframe")
-            .waitFor({ state: "visible", timeout: 30000 })
+        await getIframe(page).waitFor({ state: "visible", timeout: 30000 })
     })
 
     test("settings dialog opens", async ({ page }) => {
-        const settingsButton = page.locator('[data-testid="settings-button"]')
-        await expect(settingsButton).toBeVisible()
-        await settingsButton.click()
-
-        const dialog = page.locator('[role="dialog"]')
-        await expect(dialog).toBeVisible({ timeout: 5000 })
+        await openSettings(page)
+        // openSettings already verifies dialog is visible
     })
 
     test("language selection is available", async ({ page }) => {
-        const settingsButton = page.locator('[data-testid="settings-button"]')
-        await settingsButton.click()
+        await openSettings(page)
 
         const dialog = page.locator('[role="dialog"]')
-        await expect(dialog).toBeVisible({ timeout: 5000 })
-
-        // Should have language selector showing English
         await expect(dialog.locator('text="English"')).toBeVisible()
     })
 
     test("draw.io theme toggle exists", async ({ page }) => {
-        const settingsButton = page.locator('[data-testid="settings-button"]')
-        await settingsButton.click()
+        await openSettings(page)
 
         const dialog = page.locator('[role="dialog"]')
-        await expect(dialog).toBeVisible({ timeout: 5000 })
-
-        // Should have draw.io theme option (sketch or minimal)
         const themeText = dialog.locator("text=/sketch|minimal/i")
         await expect(themeText.first()).toBeVisible()
     })