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/chat.spec.ts

@@ -1,25 +1,21 @@
-import { expect, test } from "@playwright/test"
+import { expect, getIframe, test } from "./lib/fixtures"
 
 test.describe("Chat Panel", () => {
     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("page has interactive elements", async ({ page }) => {
-        // Verify buttons exist (settings, etc.)
         const buttons = page.locator("button")
         const count = await buttons.count()
         expect(count).toBeGreaterThan(0)
     })
 
     test("draw.io iframe is interactive", async ({ page }) => {
-        const iframe = page.locator("iframe")
+        const iframe = getIframe(page)
         await expect(iframe).toBeVisible()
 
-        // Iframe should have loaded draw.io
         const src = await iframe.getAttribute("src")
         expect(src).toBeTruthy()
     })