## 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)