Switch default output to MP3, fix AAC for iPod Nano 7G, auto-remove gray source rows after convert

- Separate AAC and MP3 ffmpeg paths: remove +global_header and +genpts from MP3
- Clamp AAC bitrate to 160 kbps (iPod Nano 7G max); warn on override
- Make mp3 the default format in UI, converter, and CLI
- Auto-remove converted source tracks from library UI (keep files on disk)
- Bulk delete_tracks with single scan+sync (progress support)
- Async duplicate removal with progress via WorkerThread

src/audio_converter.py

@@ -42,7 +42,7 @@ class AudioConverter:
     def convert_to_ipod_format(self, 
                               track_info: TrackInfo, 
                               input_file: str,
-                              format: str = "m4a", 
+                               format: str = "mp3", 
                               audio_bitrate: int = 256,
                               overwrite: bool = False) -> str:
         """
@@ -90,24 +90,27 @@ class AudioConverter:
                 logger.warning(f"Existing file is too small ({file_size} bytes), re-converting")
                 os.remove(output_file)
         
-        # iPod Nano 7 is strict about AAC parameters:
+        if format == "m4a":
+            # iPod Nano 7 AAC requirements:
             #   - AAC-LC profile only (HE-AAC causes playback issues/crashes)
             #   - 44100 Hz sample rate (firmware expects this exactly)
+            #   - Max 160 kbps for stereo (higher may cause truncation/crackle)
             #   - +global_header: required for iPod AAC decoder init
             #   - +genpts: fix timestamp gaps from web sources
-        codec = "aac" if format == "m4a" else "libmp3lame"
+            m4a_bitrate = min(audio_bitrate, 160)
+            if audio_bitrate > 160:
+                logger.warning(
+                    "Bitrate %d kbps exceeds iPod Nano 7 max (160 kbps), clamping to 160",
+                    audio_bitrate,
+                )
             cmd = [
                 "ffmpeg", "-y", "-fflags", "+genpts", "-i", input_file,
-            "-c:a", codec,
-            "-b:a", f"{audio_bitrate}k",
+                "-c:a", "aac",
+                "-b:a", f"{m4a_bitrate}k",
                 "-ar", "44100",
                 "-ac", "2",
                 "-flags", "+global_header",
             ]
-        
-        # For M4A/AAC: force LC profile and use libfdk_aac if available (best quality),
-        # otherwise fall back to built-in aac with profile flag
-        if format == "m4a":
             # Try libfdk_aac first (best compatibility)
             fdkaac_cmd = cmd + [
                 "-map", "0:a",
@@ -129,7 +132,6 @@ class AudioConverter:
                 if "libfdk_aac" in test.stdout:
                     cmd = fdkaac_cmd
                 else:
-                    # Fallback: built-in aac with LC profile
                     cmd += [
                         "-map", "0:a",
                         "-map", "0:v?",
@@ -154,7 +156,15 @@ class AudioConverter:
                     output_file,
                 ]
         else:
-            cmd += [
+            # MP3 for iPod Nano 7G: reliable, no special decoder flags needed.
+            # libmp3lame is mature and produces streams safe for hardware decoders.
+            # No +global_header — that flag is AAC-only and confuses MP3 playback.
+            cmd = [
+                "ffmpeg", "-y", "-i", input_file,
+                "-c:a", "libmp3lame",
+                "-b:a", f"{audio_bitrate}k",
+                "-ar", "44100",
+                "-ac", "2",
                 "-map", "0:a",
                 "-map", "0:t?",
                 "-id3v2_version", "3",

src/cli.py

@@ -155,7 +155,7 @@ class YouTubeToIPodCLI:
     def convert(self, 
                tracks: List[TrackInfo], 
                output_dir: str, 
-               format: str = "m4a", 
+               format: str = "mp3", 
                 quality: int = 256,
                 video: bool = False,
                 resolution: str = "640x480") -> List[Tuple[TrackInfo, str]]: