fix: correct dm channel name resolution and add channel member access check

- Fix ChannelMemberModel.get_name() to check channel tag instead of uid for dm detection
- Add fallback to channel member label when channel name is empty
- Handle self-dm edge case in ChannelMemberService.get_dm() with null join
- Update threads template to use channel name and color instead of last message user data
- Add channel membership verification in WebView to return 404 for non-members
- Pass resolved channel member name to web template instead of raw channel label

src/snek/templates/web.html

@@ -3,7 +3,7 @@
 {% block main %}
 <section class="chat-area" id="chat">
     <div class="chat-header">
-        <h2>{{ channel.label.value }}</h2>
+        <h2>{{ name }}</h2>
     </div>
     <div class="chat-messages">
         {% for message in messages %}