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/frontend-hot-path-files.txt

@@ -0,0 +1,28 @@
+# Hot-path source files for the per-file ≥70 % coverage gate (frontend).
+#
+# Mirrors `.github/hot-path-files.txt` for the Rust crates. Each entry is a
+# workspace-relative path; the gate script (`scripts/check-frontend-hot-path-
+# coverage.sh`) reads `coverage/coverage-summary.json` produced by
+# `vitest run --coverage` and warns when a listed file drops below the floor.
+#
+# Soft today (warnings only — the workflow carries `continue-on-error: true`).
+# Flip to a hard PR-blocker by removing `continue-on-error` from the
+# `frontend-tests` workflow once a few PRs have run cleanly.
+#
+# Curation rule (mirrors the backend list): a file belongs here when its
+# hot-path code dominates the file and ≥70 % is a reasonable floor. Files
+# that are partially covered today but contain large untested surface area
+# (e.g. `playerStore.ts`, the unfinished half of `previewStore.ts`) live
+# OUTSIDE the gate until their tests grow. Add them as Phase 1 coverage
+# work lands.
+#
+# Deferred from the gate, with current coverage shown for reference:
+#   - src/store/previewStore.ts          (33 % — only `_on*` + `stopPreview` covered)
+#   - src/store/playerStore.ts           (0 % — 3300-LOC monolith, Phase 2 target)
+#   - src/utils/dynamicColors.ts         (44 % — existing tests miss branches)
+#   - src/utils/shareLink.ts             (69 % — borderline, ~5 lines short)
+
+# ── utils (already at or above threshold) ────────────────────────────
+src/utils/coverArtRegisteredSizes.ts
+src/utils/serverDisplayName.ts
+src/utils/serverMagicString.ts