feat(convert): support same-format lossless re-encoding

test/audio_conversion_utils_test.dart

@@ -0,0 +1,48 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:spotiflac_android/utils/audio_conversion_utils.dart';
+
+void main() {
+  group('same-format lossless conversion', () {
+    test('allows FLAC re-encoding for quality processing', () {
+      expect(
+        canConvertAudioFormat(sourceFormat: 'FLAC', targetFormat: 'FLAC'),
+        isTrue,
+      );
+      expect(
+        canConvertAudioFormat(sourceFormat: 'flac', targetFormat: ' flac '),
+        isTrue,
+      );
+    });
+
+    test('keeps same-format lossy conversion disabled', () {
+      expect(
+        canConvertAudioFormat(sourceFormat: 'MP3', targetFormat: 'MP3'),
+        isFalse,
+      );
+      expect(
+        canConvertAudioFormat(sourceFormat: 'Opus', targetFormat: 'Opus'),
+        isFalse,
+      );
+    });
+  });
+
+  group('lossless processing options', () {
+    test('preserves both resamplers and both dither methods', () {
+      const soxrTpdf = LosslessConversionProcessing(
+        dither: 'triangular',
+        resampler: 'soxr',
+      );
+      const swrHighPass = LosslessConversionProcessing(
+        dither: 'triangular_hp',
+        resampler: 'swr',
+      );
+
+      expect(soxrTpdf.normalizedResampler, 'soxr');
+      expect(soxrTpdf.normalizedDither, 'triangular');
+      expect(soxrTpdf.hasDither, isTrue);
+      expect(swrHighPass.normalizedResampler, 'swr');
+      expect(swrHighPass.normalizedDither, 'triangular_hp');
+      expect(swrHighPass.hasDither, isTrue);
+    });
+  });
+}