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/keyboard.spec.ts

@@ -1,32 +1,22 @@
-import { expect, test } from "@playwright/test"
+import { expect, getIframe, openSettings, test } from "./lib/fixtures"
 
 test.describe("Keyboard Interactions", () => {
     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("Escape closes settings dialog", async ({ page }) => {
-        // Find settings button using aria-label or icon
-        const settingsButton = page.locator(
-            'button[aria-label*="settings"], button:has(svg[class*="settings"])',
-        )
-        await expect(settingsButton).toBeVisible()
-        await settingsButton.click()
+        await openSettings(page)
 
-        // Wait for dialog to appear
         const dialog = page.locator('[role="dialog"]')
         await expect(dialog).toBeVisible({ timeout: 5000 })
 
-        // Press Escape and verify dialog closes
         await page.keyboard.press("Escape")
         await expect(dialog).not.toBeVisible({ timeout: 2000 })
     })
 
     test("page is keyboard accessible", async ({ page }) => {
-        // Verify page has focusable elements
         const focusableElements = page.locator(
             'button, [tabindex="0"], input, textarea, a[href]',
         )