Add unit tests for AppComponent

### Task Summary

`AppComponent` (`frontend/src/app/app.component.ts`) has no `*.spec.ts` file. It is the root shell component with one piece of logic: in the constructor it sets `configLoaded = true` if reading `config.env` succeeds, or `false` if accessing it throws. It also has a `retry()` method that reloads the page. Both are quick to test by stubbing `GuiConfigService`.

### What to do (step by step)

1. Create `frontend/src/app/app.component.spec.ts` with the Apache license header.
2. Provide a stubbed `GuiConfigService` with an `env` getter you can make succeed or throw.
3. Assert `configLoaded` for both cases.
4. For `retry()`, spy on `window.location.reload` (in jsdom you may need to redefine it; see tip below) and assert it is called. If reload is awkward to spy on in the test environment, it is acceptable to skip that single assertion and note why in the PR.
5. Run `yarn test -- app.component`, then `yarn lint`.

### Cover

- `configLoaded === true` when `config.env` returns normally.
- `configLoaded === false` when accessing `config.env` throws.
- `retry()` triggers a page reload (best-effort).

### Suggested providers / mocks

```ts
import { GuiConfigService } from "src/app/common/service/gui-config.service";

// success case:
{ provide: GuiConfigService, useValue: { get env() { return {}; } } }
// failure case:
{ provide: GuiConfigService, useValue: { get env() { throw new Error("not loaded"); } } }
```

Tip for `retry()`: `const reload = vi.fn(); Object.defineProperty(window, "location", { value: { reload }, writable: true });` then assert `reload` was called.

### Task Type

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