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/model-config.spec.ts

@@ -1,27 +1,17 @@
-import { expect, test } from "@playwright/test"
+import { expect, getIframe, openSettings, test } from "./lib/fixtures"
 
 test.describe("Model Configuration", () => {
     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 and shows configuration options", async ({
         page,
     }) => {
-        // Open settings
-        const settingsButton = page.locator(
-            'button[aria-label*="settings"], button:has(svg[class*="settings"])',
-        )
-        await expect(settingsButton).toBeVisible()
-        await settingsButton.click()
+        await openSettings(page)
 
         const dialog = page.locator('[role="dialog"]')
-        await expect(dialog).toBeVisible({ timeout: 5000 })
-
-        // Settings dialog should have some configuration UI
         const buttons = dialog.locator("button")
         const buttonCount = await buttons.count()
         expect(buttonCount).toBeGreaterThan(0)