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/nodeLibraryEssentials.spec.ts

@@ -0,0 +1,110 @@
+import {
+  comfyExpect as expect,
+  comfyPageFixture as test
+} from '../fixtures/ComfyPage'
+
+test.describe('Node Library Essentials Tab', { tag: '@ui' }, () => {
+  test.beforeEach(async ({ comfyPage }) => {
+    await comfyPage.settings.setSetting('Comfy.UseNewMenu', 'Top')
+
+    // Enable the essentials feature flag via the reactive serverFeatureFlags ref.
+    // In production, this flag comes via WebSocket or remoteConfig (cloud only).
+    // The localhost test server has neither, so we set it directly.
+    await comfyPage.page.evaluate(() => {
+      window.app!.api.serverFeatureFlags.value = {
+        ...window.app!.api.serverFeatureFlags.value,
+        node_library_essentials_enabled: true
+      }
+    })
+
+    // Register a mock essential node so the essentials tab has content.
+    await comfyPage.page.evaluate(() => {
+      return window.app!.registerNodeDef('TestEssentialNode', {
+        name: 'TestEssentialNode',
+        display_name: 'Test Essential Node',
+        category: 'essentials_test',
+        input: { required: {}, optional: {} },
+        output: ['IMAGE'],
+        output_name: ['image'],
+        output_is_list: [false],
+        output_node: false,
+        python_module: 'comfy_essentials.nodes',
+        description: 'Mock essential node for testing',
+        essentials_category: 'Image Generation'
+      })
+    })
+  })
+
+  test('Node library opens via sidebar', async ({ comfyPage }) => {
+    const tabButton = comfyPage.page.locator('.node-library-tab-button')
+    await tabButton.click()
+
+    const sidebarContent = comfyPage.page.locator(
+      '.comfy-vue-side-bar-container'
+    )
+    await expect(sidebarContent).toBeVisible()
+  })
+
+  test('Essentials tab is visible in node library', async ({ comfyPage }) => {
+    const tabButton = comfyPage.page.locator('.node-library-tab-button')
+    await tabButton.click()
+
+    const essentialsTab = comfyPage.page.getByRole('tab', {
+      name: /essentials/i
+    })
+    await expect(essentialsTab).toBeVisible()
+  })
+
+  test('Clicking essentials tab shows essential node cards', async ({
+    comfyPage
+  }) => {
+    const tabButton = comfyPage.page.locator('.node-library-tab-button')
+    await tabButton.click()
+
+    const essentialsTab = comfyPage.page.getByRole('tab', {
+      name: /essentials/i
+    })
+    await essentialsTab.click()
+
+    const essentialCards = comfyPage.page.locator('[data-node-name]')
+    await expect(essentialCards.first()).toBeVisible()
+  })
+
+  test('Essential node cards have node names', async ({ comfyPage }) => {
+    const tabButton = comfyPage.page.locator('.node-library-tab-button')
+    await tabButton.click()
+
+    const essentialsTab = comfyPage.page.getByRole('tab', {
+      name: /essentials/i
+    })
+    await essentialsTab.click()
+
+    const firstCard = comfyPage.page.locator('[data-node-name]').first()
+    await expect(firstCard).toBeVisible()
+
+    const nodeName = await firstCard.getAttribute('data-node-name')
+    expect(nodeName).toBeTruthy()
+    expect(nodeName!.length).toBeGreaterThan(0)
+  })
+
+  test('Node library can switch between all and essentials tabs', async ({
+    comfyPage
+  }) => {
+    const tabButton = comfyPage.page.locator('.node-library-tab-button')
+    await tabButton.click()
+
+    const essentialsTab = comfyPage.page.getByRole('tab', {
+      name: /essentials/i
+    })
+    const allNodesTab = comfyPage.page.getByRole('tab', { name: /^all$/i })
+
+    await essentialsTab.click()
+    await expect(essentialsTab).toHaveAttribute('aria-selected', 'true')
+    const essentialCards = comfyPage.page.locator('[data-node-name]')
+    await expect(essentialCards.first()).toBeVisible()
+
+    await allNodesTab.click()
+    await expect(allNodesTab).toHaveAttribute('aria-selected', 'true')
+    await expect(essentialsTab).toHaveAttribute('aria-selected', 'false')
+  })
+})