feat: enhance CI workflow and update webapp styles

- Added Node.js setup and webapp testing steps to the CI workflow for improved integration.
- Updated HTML to link multiple CSS files for better modularity and organization of styles.
- Removed deprecated `style.css` and introduced new CSS files for base styles, calendar, day detail, hints, markers, states, and duty list to enhance maintainability and readability.
- Implemented new styles for improved presentation of duty information and user interactions.
- Added unit tests for new API functions and contact link rendering to ensure functionality and reliability.

.gitea/workflows/ci.yml

@@ -39,3 +39,14 @@ jobs:
       - name: Security check with Bandit
         run: |
           bandit -r duty_teller -ll
+
+      - name: Set up Node.js
+        uses: https://gitea.com/actions/setup-node@v4
+        with:
+          node-version: "20"
+
+      - name: Webapp tests
+        run: |
+          cd webapp
+          npm ci
+          npm run test