tests: flatten renderer mock to src/nitro-renderer.mock.ts (drop __mocks__/)

The Jest-style __mocks__/ folder added one indirection for a single
file. Move the stub to src/nitro-renderer.mock.ts at src/ root next to
test-setup.ts, drop the folder, repoint the vitest alias, and update
the lone test that imports the helpers directly (useDoorbellState).

Same behaviour, one fewer directory.

src/common/error-boundary/WidgetErrorBoundary.test.tsx

@@ -7,7 +7,7 @@ import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest';
 import { WidgetErrorBoundary } from './WidgetErrorBoundary';
 
 // `import { NitroLogger } from '@nitrots/nitro-renderer'` resolves to
-// `src/__mocks__/nitro-renderer.ts` via the alias in vitest.config.mts.
+// `src/nitro-renderer.mock.ts` via the alias in vitest.config.mts.
 // The SUT imports the same path, so both reach the same vi.fn instance.
 
 describe('WidgetErrorBoundary', () =>