feat: add dynamic channel list rendering and get_for_user service method

Implement `ChannelService.get_for_user` to fetch non-banned, non-deleted channels for a given user by iterating through channel memberships. Replace hardcoded sidebar links in `app.html` with Jinja2 template loop rendering channels from the context, enabling dynamic display of user-accessible chat rooms.

src/snek/templates/app.html

@@ -33,11 +33,9 @@
     <aside class="sidebar">
       <h2 class="no-select">Chat Rooms</h2>
       <ul>
-        <li><a class="no-select" href="#">General</a></li>
-        <li><a class="no-select" href="#">Development</a></li>
-        <li><a class="no-select" href="#">Support</a></li>
-        <li><a class="no-select" href="#">Random</a></li>
-       
+        {% for channel in channels %}
+        <li><a class="no-select" href="/web/{{channel['channel_uid']}}.html">{{channel['label']}}</a></li>
+        {% endfor %}
       </ul>
     </aside>
     {% endblock %}