feature: enable full functionality inside a Cupertino app

[csells]

currently the LlmChatView relies on Material -- update it to support Cupertino as well. A survey of the use of Material shows:

• Icons.* (and remove uses-material-design: true from pubspec.yaml)

  • Icons.add
  • Icons.send
  • Icons.attach_file
  • Icons.stop
  • Icons.mic
  • Icons.close
  • Icons.camera_alt
  • Icons.image
  • Icons.edit
  • Icons.copy

• snackbars (perhaps this would work?)

• Colors.* (moved to FatColors)

• showDialog (perhaps showCupertinoDialog and CupertinoAlertDialog?)

• TextField (CupertinoTextField)