fix: remove font_awesome_flutter (incompatible with Dart 3.12)

Replace FaIcon(FontAwesomeIcons.google) with plain Text widget.
Remove font_awesome_flutter from pubspec — package extends final
class IconData which is forbidden in Dart 3.12/Flutter 3.44.
Fix currentUser.value → currentUser (was Rx<UserModel?>, now UserModel?).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

windows/flutter/generated_plugins.cmake

@@ -3,11 +3,7 @@
 #
 
 list(APPEND FLUTTER_PLUGIN_LIST
-  cloud_firestore
   file_selector_windows
-  firebase_auth
-  firebase_core
-  firebase_storage
   geolocator_windows
   url_launcher_windows
 )