test: add assets sidebar empty-state coverage (#10595)

## Summary

Add the first user-centric Playwright coverage for the assets sidebar
empty state and introduce a small assets-specific test helper/page
object surface.

## Changes

- **What**: add `AssetsSidebarTab`, add `AssetsHelper`, and cover
generated/imported empty states in a dedicated browser spec

## Review Focus

This is intentionally a small first slice for assets-sidebar coverage.
The new helper still mocks the HTTP boundary in Playwright for now
because current OSS job history and input files are global backend
state, which makes true backend-seeded parallel coverage a separate
backend change.

Long-term recommendation: add backend-owned, user-scoped test seeding
for jobs/history and input assets so browser tests can hit the real
routes on a shared backend. Follow-up: COM-307.

Fixes COM-306

## Screenshots (if applicable)

Not applicable.

## Validation

- `pnpm typecheck:browser`
- `pnpm exec playwright test browser_tests/tests/sidebar/assets.spec.ts
--project=chromium` against an isolated preview env

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-10595-test-add-assets-sidebar-empty-state-coverage-3306d73d365081d1b34fdd146ae6c5c6)
by [Unito](https://www.unito.io)

browser_tests/tests/sidebar/assets.spec.ts

@@ -0,0 +1,30 @@
+import { expect } from '@playwright/test'
+
+import { comfyPageFixture as test } from '../../fixtures/ComfyPage'
+
+test.describe('Assets sidebar', () => {
+  test.beforeEach(async ({ comfyPage }) => {
+    await comfyPage.assets.mockEmptyState()
+    await comfyPage.setup()
+  })
+
+  test.afterEach(async ({ comfyPage }) => {
+    await comfyPage.assets.clearMocks()
+  })
+
+  test('Shows empty-state copy for generated and imported tabs', async ({
+    comfyPage
+  }) => {
+    const tab = comfyPage.menu.assetsTab
+
+    await tab.open()
+
+    await expect(tab.emptyStateTitle('No generated files found')).toBeVisible()
+    await expect(tab.emptyStateMessage).toBeVisible()
+
+    await tab.importedTab.click()
+
+    await expect(tab.emptyStateTitle('No imported files found')).toBeVisible()
+    await expect(tab.emptyStateMessage).toBeVisible()
+  })
+})