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/history.spec.ts

@@ -1,11 +1,9 @@
-import { expect, test } from "@playwright/test"
+import { expect, getIframe, test } from "./lib/fixtures"
 
 test.describe("History Dialog", () => {
     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("history button exists in UI", async ({ page }) => {