feat: add code coverage with artifact upload for unit and E2E tests

Add V8 coverage for unit tests via @vitest/coverage-v8 and Istanbul
instrumentation for E2E tests via vite-plugin-istanbul. Coverage data
is collected from browser pages after each Playwright test and merged
with nyc. CI uploads both coverage reports as GitHub Actions artifacts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -46,7 +46,14 @@ jobs:
           cache: pnpm
 
       - run: pnpm install --frozen-lockfile
-      - run: pnpm test:unit
+      - run: pnpm test:unit:coverage
+
+      - uses: actions/upload-artifact@v4
+        if: always()
+        with:
+          name: unit-coverage
+          path: coverage/unit/
+          retention-days: 14
 
   e2e-tests:
     name: E2E Tests
@@ -80,15 +87,21 @@ jobs:
         if: steps.playwright-cache.outputs.cache-hit == 'true'
         run: npx playwright install-deps chromium
 
-      - name: Build
+      - name: Build (with coverage instrumentation)
         run: pnpm build
         env:
           DATABASE_URL: ./data/test-crumbs.db
           NODE_ENV: test
+          VITE_COVERAGE: 'true'
 
       - run: pnpm test:e2e
         env:
           CI: true
+          VITE_COVERAGE: 'true'
+
+      - name: Merge E2E coverage
+        if: always()
+        run: npx nyc report
 
       - uses: actions/upload-artifact@v4
         if: always()
@@ -99,6 +112,13 @@ jobs:
             test-results/
           retention-days: 7
 
+      - uses: actions/upload-artifact@v4
+        if: always()
+        with:
+          name: e2e-coverage
+          path: coverage/e2e/
+          retention-days: 14
+
   docker-build:
     name: Docker Build
     runs-on: ubuntu-latest