test: vitest foundation with browser mode

Two projects in one root config: 'unit' (happy-dom) for stores, hooks,
and utils, and 'browser' (real Chromium via the playwright provider)
for component tests — no jsdom polyfills for EventSource, AudioContext,
or canvas. Harness pieces: createMockPlatform (spy-able Platform with
an updater emit handle), renderWithProviders (fresh QueryClient with
polling disabled + PlatformProvider), MSW worker with baseline health
handler, and a store-reset teardown covering all eight zustand stores.
Seed tests cover uiStore theme, serverStore invalidation, and an
AboutPage render in Chromium.

Requires node >= 20 (.node-version added); run with bun run test.

package.json

@@ -27,13 +27,23 @@
     "format:check": "biome format .",
     "check": "biome check .",
     "check:fix": "biome check --write .",
+    "test": "vitest run",
+    "test:watch": "vitest",
     "ci": "bun run typecheck && bun run build:web"
   },
   "devDependencies": {
     "@biomejs/biome": "2.3.12",
     "@types/node": "^20.0.0",
+    "@vitejs/plugin-react": "^6.0.5",
+    "@vitest/browser": "^4.1.10",
+    "@vitest/browser-playwright": "^4.1.10",
+    "happy-dom": "^20.11.2",
+    "msw": "^2.15.0",
+    "playwright": "^1.62.1",
     "tailwindcss": "^4.1.18",
-    "typescript": "^5.6.0"
+    "typescript": "^5.6.0",
+    "vitest": "^4.1.10",
+    "vitest-browser-react": "^2.2.0"
   },
   "engines": {
     "bun": ">=1.0.0"