feat: the library's folders are set in Admin › Library, and changing a folder moves its tracks with it, keeping their plays, likes and playlists (#6129)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/importer/importer.go

@@ -63,9 +63,10 @@ type Importer struct {
 	Scanner   Scanner
 	Catalogue Catalogue
 	Printer   Printer
-	// Roots are the library's folders (the scan paths). New music is filed
-	// under the one the settings name, or the first.
-	Roots []string
+	// Roots are the library's folders (the scan paths), read on every use
+	// because they change in Settings (#6129). New music is filed under the
+	// one the settings name, or the first.
+	Roots func() []string
 	// DataDir is where cached art lives, for tidying an artist an upgrade
 	// leaves with nothing (#5524). Empty skips it.
 	DataDir string
@@ -79,7 +80,7 @@ type Importer struct {
 
 // New returns an Importer. printer may be nil: the check then rests on
 // lengths and tags.
-func New(pool *pgxpool.Pool, scanner Scanner, cat Catalogue, printer Printer, roots []string, logger *slog.Logger) *Importer {
+func New(pool *pgxpool.Pool, scanner Scanner, cat Catalogue, printer Printer, roots func() []string, logger *slog.Logger) *Importer {
 	if logger == nil {
 		logger = slog.Default()
 	}
@@ -549,14 +550,15 @@ func (im *Importer) editions(ctx context.Context, group string) ([]Edition, erro
 
 // root is the library folder new music is filed into.
 func (im *Importer) root(ctx context.Context, q *dbq.Queries) (string, error) {
-	if len(im.Roots) == 0 {
-		return "", errors.New("there is no library folder to file new music into (MINSTREL_LIBRARY_SCAN_PATHS)")
+	roots := im.Roots()
+	if len(roots) == 0 {
+		return "", errors.New("there is no library folder to file new music into; add one in Settings → Library")
 	}
 	s, err := q.GetAcquisitionSettings(ctx)
-	if err == nil && s.ImportRoot != "" && slices.Contains(im.Roots, s.ImportRoot) {
+	if err == nil && s.ImportRoot != "" && slices.Contains(roots, s.ImportRoot) {
 		return s.ImportRoot, nil
 	}
-	return im.Roots[0], nil
+	return roots[0], nil
 }
 
 // held reads which of the planned tracks the library already has: by