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>

electron/preload.ts

@@ -13,6 +13,10 @@ contextBridge.exposeInMainWorld('mumh5Native', {
   platformInfo: () => ipcRenderer.invoke('platform:info'),
   updateTray: (state: unknown) => ipcRenderer.send('tray:update', state),
   linkPreview: (url: string) => ipcRenderer.invoke('preview:fetch', url),
+  publicServers: (url: string) => ipcRenderer.invoke('publist:fetch', url),
+  pingServer: (host: string, port: number) => ipcRenderer.invoke('publist:ping', host, port),
+  onContextMenu: (fn: (params: unknown) => void) => { ipcRenderer.on('context-menu', (_e, p) => fn(p)); },
+  editAction: (action: string, arg?: unknown) => ipcRenderer.send('edit:action', action, arg),
   onTrayAction: (fn: (action: string) => void) => { ipcRenderer.on('tray:action', (_e, a: string) => fn(a)); },
   describeCerts: (ders: Uint8Array[]) => ipcRenderer.invoke('certs:describe', ders),
   identities: {