Add Jivosite live chat widget support

Resolves #3 — widget is loaded on every page via base.html when
JIVOSITE_WIDGET_ID env var is set. Centralized Jinja2Templates instance
in web/templates_env.py with jivosite_widget_id as a global.

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

web/templates/base.html

@@ -64,6 +64,9 @@
         {% block content %}{% endblock %}
     </main>
 
+    {% if jivosite_widget_id %}
+    <script src="//code.jivosite.com/widget/{{ jivosite_widget_id }}" async></script>
+    {% endif %}
     <script src="https://cdn.jsdelivr.net/npm/bootstrap@5.3.3/dist/js/bootstrap.bundle.min.js"></script>
     <script src="https://cdn.jsdelivr.net/npm/inputmask@5.0.9/dist/inputmask.min.js"></script>
     <script>