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/services/open_exchange_service.dart

@@ -41,7 +41,7 @@ class ForexRateApiService {
       final parsed = jsonDecode(response.body) as Map<String, dynamic>;
       final error = parsed['error'] as Map<String, dynamic>?;
       final code = error?['code']?.toString().toLowerCase();
-      return code == 'rate_limit_reached';
+      return code == '104' || code == 'rate_limit_reached';
     } catch (_) {
       return false;
     }
@@ -136,6 +136,62 @@ class ForexRateApiService {
     if (apiKey == 'REPLACE_WITH_YOUR_FOREXRATE_API_KEY') {
       throw StateError('ForexRateAPI key is not configured.');
     }
-    return null;
+
+    final start = DateTime(year, 1, 1);
+    final end = DateTime(year, 12, 31);
+    final uri =
+        Uri.https('api.forexrateapi.com', '/v1/timeframe', <String, String>{
+          'api_key': apiKey,
+          'base': 'USD',
+          'currencies': 'SEK',
+          'start_date': DateFormat('yyyy-MM-dd').format(start),
+          'end_date': DateFormat('yyyy-MM-dd').format(end),
+        });
+
+    _log('Timeframe request: ${_redactedUri(uri)}');
+
+    final response = await _getWithRateLimitRetry(uri, operation: 'Timeframe');
+    if (response.statusCode != 200) {
+      _log(
+        'Timeframe endpoint unavailable for this account or request; falling back to per-day requests. status=${response.statusCode}',
+      );
+      return null;
+    }
+
+    final json = jsonDecode(response.body) as Map<String, dynamic>;
+    final success = json['success'] as bool? ?? false;
+    if (!success) {
+      _log(
+        'Timeframe success=false; falling back to per-day requests. error=${json['error']}',
+      );
+      return null;
+    }
+
+    final rates = json['rates'] as Map<String, dynamic>?;
+    if (rates == null) {
+      _log(
+        'Timeframe response missing rates; falling back to per-day requests.',
+      );
+      return null;
+    }
+
+    final results = <String, double>{};
+    for (final entry in rates.entries) {
+      final dayRates = entry.value as Map<String, dynamic>?;
+      final sek = dayRates?['SEK'];
+      if (sek is num) {
+        results[entry.key] = sek.toDouble();
+      }
+    }
+
+    if (results.isEmpty) {
+      _log(
+        'Timeframe parsed no SEK entries; falling back to per-day requests.',
+      );
+      return null;
+    }
+
+    _log('Timeframe parsed ${results.length} daily SEK quotes.');
+    return results;
   }
 }