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.

.node-version

@@ -0,0 +1 @@
+22