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/state/rent_controller.dart

@@ -2,7 +2,6 @@ import 'dart:convert';
 import 'dart:io';
 
 import 'package:flutter/foundation.dart';
-import 'package:flutter_dotenv/flutter_dotenv.dart';
 import 'package:http/http.dart' as http;
 import 'package:intl/intl.dart';
 import 'package:rental_income_tracker/models/app_settings.dart';
@@ -56,16 +55,17 @@ class RentController extends ChangeNotifier {
   }
 
   static String _exchangeRateApiKeyFromEnv() {
-    final key = dotenv.env['FOREX_RATE_API_KEY']?.trim() ?? '';
-    if (key.isEmpty) {
+    const key = String.fromEnvironment('FOREX_RATE_API_KEY');
+    final trimmed = key.trim();
+    if (trimmed.isEmpty) {
       if (kDebugMode) {
         debugPrint(
-          '[RentController] FOREX_RATE_API_KEY missing in .env, API calls will fail until set.',
+          '[RentController] FOREX_RATE_API_KEY missing from dart defines. Pass --dart-define-from-file=.env (or --dart-define) when building/running.',
         );
       }
       return 'REPLACE_WITH_YOUR_FOREXRATE_API_KEY';
     }
-    return key;
+    return trimmed;
   }
 
   final StorageService _storageService;