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.

vitest.config.ts

@@ -0,0 +1,47 @@
+import path from 'node:path';
+import react from '@vitejs/plugin-react';
+import { playwright } from '@vitest/browser-playwright';
+import { defineConfig } from 'vitest/config';
+import { changelogPlugin } from './app/plugins/changelog';
+
+const appSrc = path.resolve(__dirname, 'app/src');
+
+const shared = {
+  plugins: [react(), changelogPlugin(__dirname)],
+  resolve: {
+    alias: { '@': appSrc },
+  },
+};
+
+export default defineConfig({
+  ...shared,
+  test: {
+    projects: [
+      {
+        ...shared,
+        test: {
+          name: 'unit',
+          environment: 'happy-dom',
+          include: ['app/src/**/*.test.{ts,tsx}'],
+          exclude: ['app/src/**/*.browser.test.{ts,tsx}'],
+          setupFiles: ['app/src/test/setup.ts'],
+        },
+      },
+      {
+        ...shared,
+        publicDir: path.resolve(__dirname, 'app/src/test/public'),
+        test: {
+          name: 'browser',
+          include: ['app/src/**/*.browser.test.{ts,tsx}'],
+          setupFiles: ['app/src/test/setup.ts', 'app/src/test/setup.browser.ts'],
+          browser: {
+            enabled: true,
+            headless: true,
+            provider: playwright(),
+            instances: [{ browser: 'chromium' }],
+          },
+        },
+      },
+    ],
+  },
+});