test(frontend): vitest framework bootstrap + hot-path coverage gate (#536)

* test(frontend): vitest framework bootstrap + hot-path file coverage gate

Adds the harness for component, hook and store tests on top of the existing
util tests in src/utils/. Mirrors the backend rust-tests rollout: jsdom env,
v8 coverage, soft hot-path file gate, dedicated CI workflow.

What's in:
- vitest.config.ts: jsdom environment, v8 coverage, alias @ -> src
- src/test/setup.ts: jest-dom, @testing-library cleanup, vi.mock for
  @tauri-apps/api/{core,event} + plugin-shell, Map-backed Storage polyfill
  for Node 25 + jsdom 26 (both ship a broken native localStorage)
- src/test/mocks/tauri.ts: programmable onInvoke() / emitTauriEvent() helpers,
  auto-reset between tests
- src/test/helpers/factories.ts: makeTrack / makeTracks
- src/test/helpers/renderWithProviders.tsx: render() wrapped with
  MemoryRouter + I18nextProvider
- src/test/README.md: conventions doc (where tests go, how to mock Tauri,
  what to never mock)

Sample tests showing the patterns:
- src/components/CoverLightbox.test.tsx: component, queries by role
- src/store/previewStore.test.ts: store characterization, event handlers
  + stopPreview (startPreview deferred until the cross-store provider
  strategy is decided)

CI:
- .github/workflows/frontend-tests.yml: jobs for vitest, tsc, coverage +
  hot-path gate. coverage job carries continue-on-error: true (soft).
- .github/frontend-hot-path-files.txt: initial list (3 utils at >=70%).
  playerStore + the unfinished half of previewStore are deferred until
  Phase 1 coverage work lands.
- scripts/check-frontend-hot-path-coverage.sh: mirror of the rust gate.

npm scripts:
- test: one-shot run (unchanged)
- test⌚ vitest in watch mode
- test:coverage: v8 coverage + html / lcov / json-summary reports

57 / 57 tests pass; tsc --noEmit clean.

* chore(nix): sync npmDepsHash with package-lock.json

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

src/components/CoverLightbox.test.tsx

@@ -0,0 +1,61 @@
+import { describe, it, expect, vi } from 'vitest';
+import { screen, fireEvent } from '@testing-library/react';
+import userEvent from '@testing-library/user-event';
+import { renderWithProviders } from '@/test/helpers/renderWithProviders';
+import CoverLightbox from './CoverLightbox';
+
+describe('CoverLightbox', () => {
+  it('renders the cover image with the supplied src and alt', () => {
+    renderWithProviders(
+      <CoverLightbox src="https://example/cover.jpg" alt="Album cover" onClose={vi.fn()} />,
+    );
+
+    const img = screen.getByRole('img', { name: 'Album cover' });
+    expect(img).toHaveAttribute('src', 'https://example/cover.jpg');
+  });
+
+  it('calls onClose when the overlay is clicked', async () => {
+    const onClose = vi.fn();
+    renderWithProviders(
+      <CoverLightbox src="https://example/cover.jpg" alt="Album cover" onClose={onClose} />,
+    );
+
+    await userEvent.click(screen.getByRole('dialog'));
+
+    expect(onClose).toHaveBeenCalledOnce();
+  });
+
+  it('does not call onClose when the image itself is clicked (stops propagation)', async () => {
+    const onClose = vi.fn();
+    renderWithProviders(
+      <CoverLightbox src="https://example/cover.jpg" alt="Album cover" onClose={onClose} />,
+    );
+
+    await userEvent.click(screen.getByRole('img', { name: 'Album cover' }));
+
+    expect(onClose).not.toHaveBeenCalled();
+  });
+
+  it('calls onClose when Escape is pressed', () => {
+    const onClose = vi.fn();
+    renderWithProviders(
+      <CoverLightbox src="https://example/cover.jpg" alt="Album cover" onClose={onClose} />,
+    );
+
+    fireEvent.keyDown(window, { key: 'Escape' });
+
+    expect(onClose).toHaveBeenCalledOnce();
+  });
+
+  it('ignores other keys', () => {
+    const onClose = vi.fn();
+    renderWithProviders(
+      <CoverLightbox src="https://example/cover.jpg" alt="Album cover" onClose={onClose} />,
+    );
+
+    fireEvent.keyDown(window, { key: 'Enter' });
+    fireEvent.keyDown(window, { key: 'a' });
+
+    expect(onClose).not.toHaveBeenCalled();
+  });
+});