open_file_plus 3.4.1 uses deprecated PluginRegistry.Registrar API incompatible
with current Flutter. Replaced with a MethodChannel in MainActivity.kt that calls
FileProvider.getUriForFile() and launches the system package installer directly.
Also rework chat_input: 4px border radius on desktop, vPad 18px, 56px send button.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>