🐛 fix(frontend): make Playwright detect health endpoint failures

Reported by user 2026-05-03: the existing test
`home page loads and shows server health info` only asserted
visibility of the dashboard component, which is ALSO visible when
it shows the error state ("Error loading health: [GET] /api/healthz: 404").

So when the Go backend is down or the dev proxy misroutes /api/*,
the page shows a visible error UI but the e2e test PASSES — silent
regression risk.

This commit:

1. health.spec.ts now asserts:
   - health-info element visible (only present in healthy state)
   - health-status text equals "healthy"
   - "Error loading health" text NOT visible
2. Adds a regression test "home page surfaces health endpoint errors
   visibly" that mocks /api/healthz to 502 and asserts the error UI
   appears (this is reproducible regardless of backend state).
3. HealthDashboard.stories.ts: adds a documentation note about the
   useFetch-internal-state limitation for now (proper state-based
   stories require a future component prop refactor).

🤖 Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/components/HealthDashboard.stories.ts

@@ -6,11 +6,28 @@ const meta: Meta<typeof HealthDashboard> = {
   component: HealthDashboard,
   tags: ['autodocs'],
 }
-
 export default meta
 
 type Story = StoryObj<typeof meta>
 
+// Default story - calls real /api/healthz (works in browser if dev proxy + backend are up)
 export const Default: Story = {
   args: {},
 }
+
+// Documents the loading/error visuals so reviewers see them in Storybook
+// without needing a backend down. The component currently doesn't accept overrides
+// because it uses useFetch internally - this story is a placeholder for a future
+// refactor that exposes data/pending/error as props for testability.
+export const DocumentationStub: Story = {
+  args: {},
+  parameters: {
+    docs: {
+      description: {
+        story:
+          'Renders the same as Default. The HealthDashboard fetches /api/healthz internally, so loading/error states only appear when the backend is down or slow. ' +
+          'Future improvement: accept healthData/pending/error as props for easy state-based stories. Until then, see frontend/docs/e2e/ for screenshots of both healthy and error states.',
+      },
+    },
+  },
+}