feat: base-path support (ROOT_PATH) for serving under /portal

Links, form actions, and redirects are prefixed with the gateway path prefix so
the portal can be served at intervalsicu.farhoodlabs.com/portal (gateway strips
/portal; FastAPI root_path generates correct URLs). Connector URL is configurable.

src/intervalsicu_mcp_ui/templates/base.html

@@ -34,9 +34,9 @@
     <strong>Intervals.icu&nbsp;MCP</strong>
     {% if user %}
       <nav>
-        <a href="/account">My account</a>
-        {% if user.is_admin %}<a href="/admin">Admin</a>{% endif %}
-        <a href="/logout">Sign out</a>
+        <a href="{{ base }}/account">My account</a>
+        {% if user.is_admin %}<a href="{{ base }}/admin">Admin</a>{% endif %}
+        <a href="{{ base }}/logout">Sign out</a>
       </nav>
     {% endif %}
   </header>