Refactor environment variable handling and update build instructions

- Introduced a Makefile for building the release APK with `make build release`.
- Updated README to reflect new build command and usage of `--dart-define-from-file=.env`.
- Removed dotenv dependency and switched to using Dart's `--dart-define` for API key management.
- Adjusted ForexRateAPI key loading logic to improve error handling and clarity.

Signed-off-by: Hans Kokx <hans.d.kokx@gmail.com>

pubspec.yaml

@@ -10,7 +10,6 @@ dependencies:
   flutter:
     sdk: flutter
   file_picker: ^10.3.2
-  flutter_dotenv: ^5.2.1
   flutter_local_notifications: ^19.5.0
   flutter_timezone: ^4.1.1
   http: ^1.5.0
@@ -27,5 +26,3 @@ dev_dependencies:
 
 flutter:
   uses-material-design: true
-  assets:
-    - ./.env