fix(templates): use url_for for static assets and root_path for nav links

Fixes 404 errors for CSS/JS when served behind IIS reverse proxy with
/gomag prefix. Replaces hardcoded /static/ paths with request.url_for()
and nav links with request.scope root_path prefix.

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

api/app/templates/mappings.html

@@ -154,5 +154,5 @@
 {% endblock %}
 
 {% block scripts %}
-<script src="/static/js/mappings.js?v=7"></script>
+<script src="{{ request.url_for('static', path='js/mappings.js') }}?v=7"></script>
 {% endblock %}