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>

lib/screens/settings_screen.dart

@@ -198,7 +198,7 @@ class _SettingsScreenState extends State<SettingsScreen> {
               ),
               const SizedBox(height: 16),
               const Text(
-                'ForexRateAPI key is loaded from .env (FOREX_RATE_API_KEY).',
+                'ForexRateAPI key is loaded from FOREX_RATE_API_KEY via --dart-define-from-file=.env.',
               ),
             ],
           ),