test(web): add frontend unit test baseline (#5451)

web/test/setup.test.ts

@@ -0,0 +1,25 @@
+import { describe, expect, it, vi } from 'vitest'
+
+const capturedFetch = globalThis.fetch
+
+describe('unit-test environment', () => {
+  it('blocks real network requests before test modules are evaluated', async () => {
+    await expect(capturedFetch('https://example.invalid/')).rejects.toThrow(
+      'Real network requests are disabled in unit tests',
+    )
+  })
+
+  it('allows a test to replace fetch explicitly', async () => {
+    const replacement = vi.fn(async () => new Response('ok'))
+    vi.stubGlobal('fetch', replacement)
+
+    await expect(fetch('/test-endpoint')).resolves.toBeInstanceOf(Response)
+    expect(replacement).toHaveBeenCalledWith('/test-endpoint')
+  })
+
+  it('restores the network guard after a test replacement', async () => {
+    await expect(fetch('/must-remain-isolated')).rejects.toThrow(
+      'Real network requests are disabled in unit tests',
+    )
+  })
+})