Add channel and permission management, sounds, themes and the public server list

- Channels: create, edit, delete, link and move (drag and drop for channels and
  people), right-click on free space in the channel list
- Permission editor with inherited rules, allow/deny per permission, groups and
  members; menus grey out actions the server does not allow
- Notification sounds for connection, channel, chat, mute and push-to-talk events;
  custom files and TeamSpeak 3 style sound packs (settings.ini), stored in IndexedDB
- Color schemes: Dark, Light, Midnight, Frost and Windows 95
- Public server list with live UDP ping, fetched through Chromium's network stack
- Kick, ban and connection-lost card with Reconnect
- Right-click menus for text fields, links, images and message authors
- Formatting toolbar in the message box, shared with the description editor
- Resizable channel list, menu headers, registration from the self menu only
- Fix: client-side pacing now mirrors Murmur's whole-second leaky bucket exactly,
  so quick actions are no longer dropped silently

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

src/lib/ui.svelte.ts

@@ -25,7 +25,12 @@ class Ui {
   wizard = $state<'setup' | 'add' | null>(null);
   // Quick audio popover from right-clicking mute (input) or deafen (output)
   quickAudio = $state<{ kind: 'input' | 'output'; x: number; y: number } | null>(null);
-  settingsTab = $state<'voice' | 'files' | 'identities'>('voice');
+  settingsTab = $state<'voice' | 'sounds' | 'appearance' | 'files' | 'identities'>('voice');
+  publicServers = $state(false);
+  // Add-server dialog, optionally prefilled (from the public list)
+  addServer = $state<{ host?: string; port?: number; label?: string } | null>(null);
+  // Channel dialog: create a subchannel of `parent`, or edit channel `id`
+  channelDialog = $state<{ mode: 'create'; parent: number } | { mode: 'edit'; id: number; tab?: 'general' | 'permissions' | 'groups' } | null>(null);
   // Session whose connection information is shown
   userInfo = $state<number | null>(null);
   // Set when a panel was opened on a layout where it is an overlay