fix: replace open_file_plus with native Kotlin MethodChannel for APK install

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>

pubspec.yaml

@@ -57,7 +57,6 @@ dependencies:
   # File handling
   file_picker: ^8.1.7
   desktop_drop: ^0.4.4
-  open_file_plus: ^3.4.1
 
   # Media playback
   media_kit: ^1.1.11