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.lock

@@ -757,14 +757,6 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "9.3.0"
-  open_file_plus:
-    dependency: "direct main"
-    description:
-      name: open_file_plus
-      sha256: "01ad7011d46d253aa22e5f0bc7345ca49cfe652c50ae9b0af945491c1728a8c3"
-      url: "https://pub.dev"
-    source: hosted
-    version: "3.4.1+1"
   package_config:
     dependency: transitive
     description: