fix: route FLAC clip extraction through temp file to fix corrupted headers

FLAC output to pipe (pipe:1) produces broken STREAMINFO headers because
FFmpeg cannot seek back to finalize the total sample count, min/max frame
sizes, and MD5 checksum. This caused exported FLAC files to have zeroed
metadata fields and corrupted audio on playback.

Fix by adding FormatFLAC to requiresSeekableOutput so clip extraction
uses a temporary file, same as AAC and ALAC already do.

Author: tphakala

internal/myaudio/ffmpeg_clip.go

@@ -45,10 +45,13 @@ var supportedClipFormats = map[string]bool{
 
 // requiresSeekableOutput lists formats whose muxers cannot write to a pipe.
 // MP4-based containers (AAC → mp4, ALAC → ipod) need seekable output to
-// write the moov atom, so we route them through a temporary file.
+// write the moov atom. FLAC needs seekable output to finalize the STREAMINFO
+// header (total sample count, min/max frame sizes, MD5 checksum). Without it,
+// these fields are zeroed and players may reject or misinterpret the file.
 var requiresSeekableOutput = map[string]bool{
 	FormatAAC:  true,
 	FormatALAC: true,
+	FormatFLAC: true,
 }
 
 // IsSupportedClipFormat returns true if the format is supported for clip extraction.