Move InMemoryJobsBackend mock to devtools once ComfyUI jobs API is user-scoped

## Context

The Playwright test fixture `browser_tests/fixtures/helpers/InMemoryJobsBackend.ts` (introduced in #10630) duplicates backend logic — filtering, sorting, and pagination for `/api/jobs` — directly in the frontend test suite. This creates an ongoing FE/BE drift risk: any change to the real API's query behavior (new filter params, sort keys, pagination semantics) must be manually mirrored in this mock or tests silently diverge from production behavior.

## Desired outcome

Once ComfyUI makes the jobs API user-scoped, `InMemoryJobsBackend` should be extracted from `browser_tests/` and moved into a shared devtools package, eliminating the drift risk and making the mock a first-class artifact maintained alongside the real backend contract.

## Action items

- [ ] Track ComfyUI backend work to make the jobs API user-scoped
- [ ] Move `InMemoryJobsBackend` (and related seeding helpers) to the appropriate devtools package once the backend prerequisite lands
- [ ] Update `browser_tests/` imports to reference the devtools package instead of the local helper

## References

- PR introducing the mock: #10630
- Relevant review discussion: https://github.com/Comfy-Org/ComfyUI_frontend/pull/10630#discussion_r3041586310
- Requested by: @DrJKL

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-11203-Move-InMemoryJobsBackend-mock-to-devtools-once-ComfyUI-jobs-API-is-user-scoped-3416d73d365081458bc2cd4e917d87a5) by [Unito](https://www.unito.io)