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/graphCanvasMenu.spec.ts

@@ -60,6 +60,15 @@ test.describe('Graph Canvas Menu', { tag: ['@screenshot', '@canvas'] }, () => {
     await comfyPage.nextFrame()
   })
 
+  test('Fit view button is present and clickable', async ({ comfyPage }) => {
+    const fitViewButton = comfyPage.page
+      .locator('button')
+      .filter({ has: comfyPage.page.locator('.icon-\\[lucide--focus\\]') })
+    await expect(fitViewButton).toBeVisible()
+    await fitViewButton.click()
+    await comfyPage.nextFrame()
+  })
+
   test('Zoom controls popup opens and closes', async ({ comfyPage }) => {
     // Find the zoom button by its percentage text content
     const zoomButton = comfyPage.page.locator('button').filter({