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>

.github/workflows/frontend-tests.yml

@@ -0,0 +1,85 @@
+name: frontend-tests
+
+on:
+  pull_request:
+    branches: [main]
+    paths:
+      - 'src/**'
+      - 'package.json'
+      - 'package-lock.json'
+      - 'vitest.config.ts'
+      - 'vite.config.ts'
+      - 'tsconfig.json'
+      - '.github/workflows/frontend-tests.yml'
+      - '.github/frontend-hot-path-files.txt'
+      - 'scripts/check-frontend-hot-path-coverage.sh'
+  push:
+    branches: [main]
+    paths:
+      - 'src/**'
+      - 'package.json'
+      - 'package-lock.json'
+      - 'vitest.config.ts'
+      - 'vite.config.ts'
+      - 'tsconfig.json'
+      - '.github/workflows/frontend-tests.yml'
+      - '.github/frontend-hot-path-files.txt'
+      - 'scripts/check-frontend-hot-path-coverage.sh'
+  workflow_dispatch:
+
+permissions:
+  contents: read
+
+jobs:
+  test:
+    name: vitest run
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: actions/checkout@v5
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          cache: 'npm'
+      - run: npm ci
+      - name: vitest
+        run: npm test
+
+  typecheck:
+    name: tsc --noEmit
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: actions/checkout@v5
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          cache: 'npm'
+      - run: npm ci
+      - name: tsc
+        run: npx tsc --noEmit
+
+  coverage:
+    name: vitest --coverage (baseline + hot-path file gate)
+    # Two-layer gate: the script exits 1 when any listed file drops below the
+    # threshold (warning annotations show in the PR checks panel), but
+    # `continue-on-error: true` keeps it from BLOCKING merges. Drop the flag
+    # to flip the gate hard once we've watched a few PRs run cleanly.
+    runs-on: ubuntu-24.04
+    continue-on-error: true
+    steps:
+      - uses: actions/checkout@v5
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '20'
+          cache: 'npm'
+      - name: install jq
+        run: sudo apt-get update && sudo apt-get install -y jq
+      - run: npm ci
+      - name: vitest run --coverage
+        run: npx vitest run --coverage
+      - name: hot-path file coverage soft gate
+        run: bash scripts/check-frontend-hot-path-coverage.sh
+      - uses: actions/upload-artifact@v4
+        with:
+          name: frontend-coverage-lcov
+          path: coverage/lcov.info
+          if-no-files-found: error