v2.1: Architectural cleanup

Updates:

• Upgraded to the latest Flutter build.
• Cleaned up the code and modularized it somewhat, ready for an upcoming UI refresh.
• Republished it to prepare for Codemagic.

v1.2: The FAB Update

Updates:

• Brought in FloatingActionButton.extended()
• Added auto_size_text to avoid text overflow
• Updated packages to the latest versions
• Compiled with Flutter test release
• Added a gratuitous icon to the app bar
• Tweaked Gradle instructions to match latest guidance
• Took advantage of Dart int/double inference