test: add FeatureFlagHelper and QueueHelper for E2E test infrastructure (#9554)

## Summary

Add 2 reusable test helpers for Playwright E2E tests, integrated into
the ComfyPage fixture. These provide standardized patterns for mocking
feature flags and queue state across all E2E tests.

## Changes

- **`FeatureFlagHelper.ts`** — manage localStorage `ff:` prefixed
feature flags (`seedFlags` for init-time, `setFlags` for runtime) and
mock `/api/features` route
- **`QueueHelper.ts`** — mock `/api/queue` and `/api/history` routes
with configurable running/pending counts and success/error job entries
- **`ComfyPage.ts`** — integrate both helpers as
`comfyPage.featureFlags` and `comfyPage.queue`

## Review Focus

- Helper API design: are `seedFlags`/`setFlags`/`mockServerFeatures` the
right abstractions for feature flag testing?
- Queue mock fidelity: does the mock history shape match real ComfyUI
API responses closely enough?
- These are test-only infrastructure — no production code changes.

## Stack

This is the base PR for the Playwright E2E coverage stack. Waves 1-4 all
branch from this and can merge independently once this lands:
- **→ This PR**: Test infrastructure helpers
- #9555: Toasts, error overlay, selection toolbox, linear mode,
selection rectangle
- #9556: Node search, bottom panel, focus mode, job history, side panel
- #9557: Errors tab, node headers, queue notifications, settings sidebar
- #9558: Minimap, widget copy, floating menus, node library essentials

---------

Co-authored-by: GitHub Action <action@github.com>

browser_tests/tests/widgetCopyButton.spec.ts

@@ -0,0 +1,45 @@
+import {
+  comfyExpect as expect,
+  comfyPageFixture as test
+} from '../fixtures/ComfyPage'
+
+test.describe('Widget copy button', { tag: '@ui' }, () => {
+  test.beforeEach(async ({ comfyPage }) => {
+    await comfyPage.settings.setSetting('Comfy.UseNewMenu', 'Top')
+    await comfyPage.settings.setSetting('Comfy.VueNodes.Enabled', true)
+    await comfyPage.setup()
+
+    // Add a PreviewAny node which has a read-only textarea with a copy button
+    await comfyPage.page.evaluate(() => {
+      const node = window.LiteGraph!.createNode('PreviewAny')
+      window.app!.graph.add(node)
+    })
+
+    await comfyPage.vueNodes.waitForNodes()
+  })
+
+  test('Copy button has correct aria-label', async ({ comfyPage }) => {
+    const copyButton = comfyPage.page
+      .locator('[data-node-id] button[aria-label]')
+      .filter({ has: comfyPage.page.locator('.icon-\\[lucide--copy\\]') })
+      .first()
+    await expect(copyButton).toBeAttached()
+    await expect(copyButton).toHaveAttribute('aria-label', /copy/i)
+  })
+
+  test('Copy button exists within textarea widget group container', async ({
+    comfyPage
+  }) => {
+    const container = comfyPage.page
+      .locator('[data-node-id] div.group:has(textarea[readonly])')
+      .first()
+    await expect(container).toBeVisible()
+    await container.hover()
+    await comfyPage.nextFrame()
+
+    const copyButton = container.locator('button').filter({
+      has: comfyPage.page.locator('.icon-\\[lucide--copy\\]')
+    })
+    await expect(copyButton.first()).toBeAttached()
+  })
+})