Fix flaky E2E tests: layers panel, zoom-to-fit, snapshot stability

- layers-panel: navigate to /demo so demo layers exist
- app-menu zoom-to-fit: zoom in first so zoom-to-fit has something to change
- playwright.config: force workers=1 and colorScheme=dark for consistent
  WebGL/SwiftShader snapshot rendering across runs
- Update empty-canvas snapshot for current dark-mode canvas appearance

playwright.config.ts

@@ -3,6 +3,7 @@ import { defineConfig } from '@playwright/test'
 export default defineConfig({
   testDir: './tests',
   timeout: 15_000,
+  workers: 1,
   expect: {
     toHaveScreenshot: {
       maxDiffPixelRatio: 0.01,
@@ -17,6 +18,7 @@ export default defineConfig({
     baseURL: 'http://localhost:1420',
     viewport: { width: 1280, height: 800 },
     deviceScaleFactor: 2,
+    colorScheme: 'dark',
     launchOptions: {
       args: ['--enable-unsafe-swiftshader']
     }
@@ -24,7 +26,8 @@ export default defineConfig({
   projects: [
     {
       name: 'openpencil',
-      testDir: './tests/e2e'
+      testDir: './tests/e2e',
+      fullyParallel: false
     },
     {
       name: 'figma',

tests/e2e/app-menu.spec.ts

@@ -120,7 +120,12 @@ test('Duplicate via Edit menu works', async () => {
 })
 
 test('Zoom to fit via View menu works', async () => {
+  await page.locator('[role="menubar"] [role="menuitem"]', { hasText: 'View' }).click()
+  await page.locator('[role="menu"] [role="menuitem"]', { hasText: 'Zoom in' }).click()
+  await canvas.waitForRender()
+
   const zoomBefore = await page.evaluate(() => window.__OPEN_PENCIL_STORE__!.state.zoom)
+  expect(zoomBefore).toBeGreaterThan(1)
 
   await page.locator('[role="menubar"] [role="menuitem"]', { hasText: 'View' }).click()
   await page.locator('[role="menu"] [role="menuitem"]', { hasText: 'Zoom to fit' }).click()

tests/e2e/layers-panel.spec.ts

@@ -9,7 +9,7 @@ test.describe.configure({ mode: 'serial' })
 
 test.beforeAll(async ({ browser }) => {
   page = await browser.newPage()
-  await page.goto('/')
+  await page.goto('/demo')
   canvas = new CanvasHelper(page)
   await canvas.waitForInit()
 })