🐛 fix(frontend): apply server:false + route.fulfill to health spec

Closes the debt left by PR #40 (api/info + footer): the existing
tests/e2e/health.spec.ts happy path still hit the dev-proxy issue
that PR #40 worked around for the new app-footer spec.

Same fix applied to HealthDashboard.vue and the spec:
- HealthDashboard.vue: useFetch( ..., { server: false }) — fetch
  client-side only so Playwright route mocks apply (was failing on
  SSR through the dev proxy).
- health.spec.ts: happy path now uses page.route to mock /api/healthz
  with a 200 success body, matching the pattern in app-footer.spec.ts.
  The error scenario already used route mocking; unchanged.

The integration with the real backend remains covered by the BDD
scenario in features/health/health.feature (server-side, no proxy
in the loop).

Full Playwright suite (4 tests) now passes locally.

frontend/tests/e2e/health.spec.ts

@@ -1,6 +1,24 @@
 import { test, expect } from '@playwright/test'
 
+// Both specs mock /api/healthz so they decouple from the dev-proxy plumbing.
+// The integration with the real backend is covered by the BDD scenario in
+// features/health/health.feature (server-side, no frontend proxy in the loop).
+// Same approach as tests/e2e/app-footer.spec.ts (PR #40) - applied here to
+// close the debt left by that PR's out-of-scope follow-up note.
+
 test('home page loads and shows healthy server state', async ({ page }) => {
+  await page.route('**/api/healthz', (route) => {
+    route.fulfill({
+      status: 200,
+      contentType: 'application/json',
+      body: JSON.stringify({
+        status: 'healthy',
+        version: '1.4.0',
+        uptime_seconds: 8042,
+        timestamp: '2026-05-05T08:00:00Z',
+      }),
+    })
+  })
   await page.goto('/')
   await expect(page.getByTestId('health-dashboard')).toBeVisible()
   const heading = page.getByRole('heading', { name: /dance-lessons-coach/i })