ci: add combined coverage gate to unit-tests job

Runs test:coverage (server v8 + client Istanbul) after tests, hard-gates
on both 80% branch thresholds, and uploads coverage/ as an artifact.

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

.gitea/workflows/ci.yml

@@ -42,6 +42,19 @@ jobs:
         env:
           TZ: Europe/Berlin
 
+      - name: Run coverage (server + client)
+        run: npm run test:coverage
+        working-directory: frontend
+        env:
+          TZ: Europe/Berlin
+
+      - name: Upload coverage reports
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: coverage-reports
+          path: frontend/coverage/
+
       - name: Build frontend
         run: npm run build
         working-directory: frontend