Text channels between mumh5 users; drag channels in front of or behind another

Text channels are real Mumble channels with a marker in the description.
Their messages travel as plugin data to everyone on the server, are kept
by the clients and handed to people who connect later.

Channels can be dropped in front of or behind another channel, with a
line where they land. A description preview no longer sticks after a drop.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ui/Chat.svelte

@@ -21,6 +21,7 @@
   const logFixed = $derived(streamAbove && logHeight >= LOG_MIN);
 
   const isDm = $derived(session.view.startsWith('dm:'));
+  const isText = $derived(!isDm && session.isText(Number(session.view.slice(3))));
   const title = $derived(session.status === 'connected' ? session.viewTitle() : 'mumh5');
   const topic = $derived.by(() => {
     if (isDm || session.status !== 'connected') return '';
@@ -71,7 +72,7 @@
       <div class="empty">
         {#if session.status === 'connected'}
           <h2>{isDm ? `Direct messages with ${title}` : `Welcome to #${title}`}</h2>
-          <p>{isDm ? 'Messages here are only visible to the two of you.' : 'Messages go to everyone in this channel. Drop files here to share them.'}</p>
+          <p>{isDm ? 'Messages here are only visible to the two of you.' : isText ? 'A text channel: everyone on this server who uses mumh5 can read and write here, from any channel. Other Mumble clients do not see these messages.' : 'Messages go to everyone in this channel. Drop files here to share them.'}</p>
         {:else if session.status === 'connecting'}
           <h2>Connecting...</h2>
         {:else}