Fix lossy copy conversion failing on FLACs with embedded cover art

Added -vn flag to all codec ffmpeg commands (MP3, Opus, AAC) to strip
video/image streams during conversion. Embedded cover art in FLAC
files caused ffmpeg to fail when the output muxer couldn't handle
the image stream, producing 0KB output files. Cover art is
re-embedded afterwards by Mutagen.

web_server.py

@@ -18402,10 +18402,12 @@ def _create_lossy_copy(final_path):
         bitrate = '256'
 
     # Codec configuration: (ffmpeg_codec, extension, quality_label, extra_args)
+    # -vn strips video/image streams (embedded cover art) which can cause
+    # conversion failures when the output muxer can't handle image streams
     codec_map = {
-        'mp3':  ('libmp3lame', '.mp3',  f'MP3-{bitrate}',  ['-id3v2_version', '3']),
-        'opus': ('libopus',    '.opus', f'OPUS-{bitrate}',  ['-map', '0:a', '-vbr', 'on']),
-        'aac':  ('aac',        '.m4a',  f'AAC-{bitrate}',   ['-movflags', '+faststart']),
+        'mp3':  ('libmp3lame', '.mp3',  f'MP3-{bitrate}',  ['-vn', '-id3v2_version', '3']),
+        'opus': ('libopus',    '.opus', f'OPUS-{bitrate}',  ['-vn', '-map', '0:a', '-vbr', 'on']),
+        'aac':  ('aac',        '.m4a',  f'AAC-{bitrate}',   ['-vn', '-movflags', '+faststart']),
     }
 
     if codec not in codec_map: