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>

README.md

@@ -41,7 +41,7 @@ mumh5 keeps the foundation and replaces the experience.
 | Look | One Qt style (plus skins) | Five built-in color schemes, including Windows 95 |
 | Channel and permission editing | Dialogs, drag and drop | Same power: create, edit, link, drag and drop, rule and group editor |
 
-**Where the desktop client is still ahead, today:** system-wide push-to-talk, whisper and shout, positional audio, the in-game overlay, recording, and the ban list and registered-user editors. These are on the roadmap below.
+**Where the desktop client is still ahead, today:** system-wide push-to-talk, whisper and shout, positional audio, the in-game overlay, recording, and the ban list and registered-user editors. Push-to-talk and whisper are on the roadmap below.
 
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@@ -281,6 +281,8 @@ Without an upload host, mumh5 still sends images, scaled to fit the server's lim
 
 - System-wide push to talk
 - Whisper and shout
+- Text channels: private ones, and history while nobody is online (needs a helper on the server)
+- A second, mobile-first interface with the same features in a different layout
 - Rich chat between mumh5 users: replies, reactions, edits, typing indicators
 - Screen sharing: a media server for large rounds (today every sender uploads once per viewer, up to 8)
 - Release builds for all platforms