test(e2e): add Playwright smoke suite + axe-core a11y gate + CI (#146)

Builds out the foundation for end-to-end tests against the production
build: Playwright with desktop + mobile projects, axe-core for WCAG
2.1 AA gating, and a GitHub Actions workflow that caches the browser
download and uploads the HTML report on failure.

What lands now (public surfaces only — no DB seed required):

  e2e/version.spec.ts          /api/version returns shape, no auth
  e2e/auth-redirect.spec.ts    proxy gate bounces / → /auth/login etc.
  e2e/login.spec.ts            login form renders + autoComplete hints
                               + invalid-credential error surfaces
  e2e/locale-switch.spec.ts    DE/EN cookie switch, no raw i18n keys
                               leak into the DOM
  e2e/a11y.spec.ts             axe-core fails on serious or critical
                               WCAG 2.1 A/AA violations on /auth/login

The four authenticated flow specs from the v1.4.1 plan
(quick-entry, doctor-report, settings-roundtrip, test-buttons,
onboarding) need a seeded test user; they're tracked as a follow-up
because the seeding pipeline + WebAuthn polyfill they require warrant
their own PR. The smoke specs above cover the proxy, the login surface,
the version healthcheck endpoint, and the locale machinery — i.e.
every path a v1.4.X regression could realistically break without
deeper UAT.

CI workflow (.github/workflows/e2e.yml):
  - postgres:16-alpine service container
  - prisma migrate deploy seeds an empty schema
  - playwright install --with-deps chromium (cached)
  - next build, then `playwright test` against `next start`
  - playwright-report uploaded as an artifact on failure

Quality gates:
  - pnpm typecheck — clean
  - pnpm test — 658/658 pass (vitest excludes e2e/**)
  - pnpm lint — 0 errors

.github/workflows/e2e.yml

@@ -0,0 +1,83 @@
+name: e2e
+
+on:
+  pull_request:
+    branches: [main]
+  push:
+    branches: [main]
+
+# Cancel in-progress runs when a new commit lands on the same PR.
+concurrency:
+  group: e2e-${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  e2e:
+    runs-on: ubuntu-latest
+    timeout-minutes: 20
+    services:
+      postgres:
+        image: postgres:16-alpine
+        env:
+          POSTGRES_USER: healthlog
+          POSTGRES_PASSWORD: healthlog
+          POSTGRES_DB: healthlog
+        ports:
+          - 5432:5432
+        options: >-
+          --health-cmd "pg_isready -U healthlog"
+          --health-interval 5s
+          --health-timeout 5s
+          --health-retries 10
+
+    env:
+      DATABASE_URL: postgresql://healthlog:healthlog@localhost:5432/healthlog?schema=public
+      ENCRYPTION_KEYS: '{"v1":"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa"}'
+      ENCRYPTION_ACTIVE_KEY_ID: v1
+      API_TOKEN_HMAC_KEY: ci-hmac-key-32-bytes-min-padding-x
+      AUTH_RP_NAME: HealthLog
+      AUTH_RP_ID: localhost
+      AUTH_RP_ORIGIN: http://localhost:3000
+      NODE_ENV: production
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: pnpm/action-setup@v4
+        with:
+          version: 10
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: pnpm
+
+      - run: pnpm install --frozen-lockfile
+
+      - run: pnpm db:generate
+
+      - run: pnpm db:migrate:deploy
+
+      - name: Cache Playwright browsers
+        uses: actions/cache@v4
+        with:
+          path: ~/.cache/ms-playwright
+          key: playwright-${{ runner.os }}-${{ hashFiles('pnpm-lock.yaml') }}
+          restore-keys: |
+            playwright-${{ runner.os }}-
+
+      - run: pnpm exec playwright install --with-deps chromium
+
+      - run: pnpm exec next build
+
+      - run: pnpm exec playwright test
+        env:
+          E2E_BASE_URL: http://localhost:3000
+
+      - name: Upload Playwright HTML report on failure
+        if: failure()
+        uses: actions/upload-artifact@v4
+        with:
+          name: playwright-report
+          path: playwright-report/
+          retention-days: 7