Add progress callback for long-running conversions

[sjoenk]

Problem

Long-running audio conversions (especially on older/slower devices) provide no feedback to the caller. Users have no way of knowing whether the conversion is 10% or 90% complete, leading to a poor user experience.

Proposed Solution

Add an optional progress callback that reports decoding progress as a percentage (0.0 to 1.0):

Dart API

await AudioDecoder.convertToWav(
  inputPath,
  outputPath,
  onProgress: (double progress) {
    print('${(progress * 100).toStringAsFixed(0)}%');
  },
);

Implementation Approach

• Use MediaExtractor.getSampleTime() relative to the total duration (from MediaFormat.KEY_DURATION) to calculate progress
• Report progress via an EventChannel or periodic method channel callbacks
• Throttle updates to avoid excessive cross-platform communication (e.g., report every 1-5%)

Expected Impact

• Better UX — users see that the conversion is progressing
• Low implementation effort — duration and current position are already available from the Android media APIs

Affected Files

• lib/audio_decoder.dart (add onProgress parameter)
• lib/audio_decoder_platform_interface.dart (platform interface update)
• lib/audio_decoder_method_channel.dart (EventChannel or callback handling)
• android/src/main/kotlin/nl/silversoft/audio_decoder/AudioDecoderPlugin.kt (progress reporting)
• ios/Classes/AudioDecoderPlugin.swift (progress reporting)