feat(16-02): add go-flac dependencies and implement FLAC tag writer

- Add go-flac/go-flac/v2, flacvorbis/v2, flacpicture/v2 dependencies
- Create tagwriter.go with TagChanges type, field constants, format detection, MIME detection
- Create flac.go with writeFlacTags using Vorbis Comments + PICTURE blocks + AtomicWrite
- Implement replaceVorbisComment helper for case-insensitive field replacement
- Handle cover art add/replace/clear via PICTURE metadata blocks

go.mod

@@ -65,6 +65,7 @@ require (
 	github.com/bep/debounce v1.2.1 // indirect
 	github.com/bkielbasa/cyclop v1.2.3 // indirect
 	github.com/blizzy78/varnamelen v0.8.0 // indirect
+	github.com/bogem/id3v2/v2 v2.1.4 // indirect
 	github.com/bombsimon/wsl/v4 v4.7.0 // indirect
 	github.com/bombsimon/wsl/v5 v5.6.0 // indirect
 	github.com/breml/bidichk v0.3.3 // indirect
@@ -112,6 +113,9 @@ require (
 	github.com/gabriel-vasile/mimetype v1.4.12 // indirect
 	github.com/ghostiam/protogetter v0.3.20 // indirect
 	github.com/go-critic/go-critic v0.14.3 // indirect
+	github.com/go-flac/flacpicture/v2 v2.0.2 // indirect
+	github.com/go-flac/flacvorbis/v2 v2.0.2 // indirect
+	github.com/go-flac/go-flac/v2 v2.0.4 // indirect
 	github.com/go-git/gcfg v1.5.1-0.20230307220236-3a3c6141e376 // indirect
 	github.com/go-git/go-billy/v5 v5.6.2 // indirect
 	github.com/go-git/go-git/v5 v5.13.2 // indirect