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_env.py

@@ -0,0 +1,6 @@
+from fastapi.templating import Jinja2Templates
+
+from web.config import settings
+
+templates = Jinja2Templates(directory="web/templates")
+templates.env.globals["jivosite_widget_id"] = settings.JIVOSITE_WIDGET_ID