feat: cap bit depth/sample rate on lossless conversion + WAV/AIFF

- LosslessConversionQuality model with bit depth/sample rate caps,
  applied only when they reduce source quality
- FFmpegService probes sample rate and appends codec-specific args
  (-ar, -sample_fmt, -bits_per_raw_sample) for FLAC/ALAC/WAV/AIFF
- Batch + single-track convert sheets expose quality cap options
- Persist real converted bit depth/sample rate to history/library DB
- track_metadata: recognize and convert to WAV/AIFF targets
- convertedAudioQualityLabel reflects actual output quality

lib/services/library_database.dart

@@ -1735,6 +1735,8 @@ class LibraryDatabase {
     required String newFilePath,
     required String targetFormat,
     required String bitrate,
+    int? bitDepth,
+    int? sampleRate,
   }) async {
     final db = await database;
     final stat = await fileStat(newFilePath);
@@ -1755,6 +1757,10 @@ class LibraryDatabase {
         normalizedFormat == 'opus' ||
         normalizedFormat == 'aac') {
       updated['bitDepth'] = null;
+      updated['sampleRate'] = null;
+    } else {
+      updated['bitDepth'] = bitDepth ?? item.bitDepth;
+      updated['sampleRate'] = sampleRate ?? item.sampleRate;
     }
 
     await db.transaction((txn) async {