add unit test coverage for PanelService

### Task Summary

Add `panel.service.spec.ts` covering `PanelService`, a tiny event bus that exposes two `Subject` streams (`closePanelStream`, `resetPanelStream`) with `closePanels()` / `resetPanels()` triggers.

## Background

`frontend/src/app/workspace/service/panel/panel.service.ts` is a dependency-free service (instantiable directly via `new PanelService()`), so no `TestBed` wiring is required.

```ts
get resetPanelStream() { return this.resetPanelSubject.asObservable(); }
resetPanels() { this.resetPanelSubject.next(); }
get closePanelStream() { return this.closePanelSubject.asObservable(); }
closePanels() { this.closePanelSubject.next(); }
```

## Behavior to pin

| Scenario | Contract |
| --- | --- |
| subscribe `resetPanelStream`, call `resetPanels()` | subscriber fires exactly once |
| subscribe `closePanelStream`, call `closePanels()` | subscriber fires exactly once |
| stream independence | calling `resetPanels()` does not emit on `closePanelStream`, and vice-versa |

Subscribe to the stream, invoke the trigger, and assert the emission count (e.g. increment a counter in the subscription). Follow `size-formatter.util.spec.ts` for the plain `describe`/`it`/`expect` style (`ng test`).

## Scope

- New spec: `frontend/src/app/workspace/service/panel/panel.service.spec.ts`.
- No production-code changes.

### Task Type

- [ ] Refactor / Cleanup
- [ ] DevOps / Deployment / CI
- [x] Testing / QA
- [ ] Documentation
- [ ] Performance
- [ ] Other

Read frontend/src/app/workspace/service/panel/panel.service.ts and follow the plain describe/it/expect style in size-formatter.util.spec.ts. Add frontend/src/app/workspace/service/panel/panel.service.spec.ts, then run ng test. Done means resetPanels() and closePanels() each emit once on their own stream and never emit on the other stream.

Written by the indexing model from the issue text.