Add release builds, classic and stacked layouts

- electron-builder packaging: AppImage, deb and tar.gz for Linux, NSIS
  installer for Windows, app icon, desktop entry
- Gitea workflow builds Linux and Windows on version tags and attaches
  the files to the release
- Layout setting in Appearance: Modern, Classic (chat left, channels
  right) and Stacked (channels on top, chat below, resizable split),
  like the desktop Mumble client
- No message box while not connected; Manage identities on the welcome
  page
- Public list: a list that arrives after the source changed no longer
  replaces the new one

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

electron/main.ts

@@ -136,6 +136,8 @@ function createWindow() {
     minHeight: 480,
     backgroundColor: '#111214',
     title: 'mumh5',
+    // Window and taskbar icon on Linux (Windows and macOS take it from the package)
+    icon: path.join(__dirname, '../dist/icon.png'),
     autoHideMenuBar: true,
     webPreferences: {
       preload: path.join(__dirname, 'preload.cjs'),