Use file name for unknown tag title retrieval.

When taglib get empty or null title from file/album, use file name or
folder name as the tag_title field. It could be confuse to user, but at
leaset it is better than tracks/albums can't be search in ID3 mode API.

scanner/scanner.go

@@ -465,8 +465,9 @@ func populateTrack(tx *db.DB, album *db.Album, track *db.Track, trags tagcommon.
 	track.Size = size
 	track.AlbumID = album.ID
 
-	track.TagTitle = trags.Title()
-	track.TagTitleUDec = decoded(trags.Title())
+	tagTitle := tagcommon.MustTitle(trags)
+	track.TagTitle = tagTitle
+	track.TagTitleUDec = decoded(tagTitle)
 	track.TagTrackArtist = tagcommon.MustArtist(trags)
 	track.TagTrackNumber = trags.TrackNumber()
 	track.TagDiscNumber = trags.DiscNumber()

tags/tagcommon/tagcommmon.go

@@ -2,6 +2,7 @@ package tagcommon
 
 import (
 	"errors"
+	"path"
 )
 
 var ErrUnsupported = errors.New("filetype unsupported")
@@ -33,19 +34,31 @@ type Info interface {
 
 	Length() int
 	Bitrate() int
+
+	AbsPath() string
 }
 
 const (
-	FallbackAlbum  = "Unknown Album"
 	FallbackArtist = "Unknown Artist"
 	FallbackGenre  = "Unknown Genre"
 )
 
+func MustTitle(p Info) string {
+	if r := p.Title(); r != "" {
+		return r
+	}
+
+	// return the file name for title name
+	return path.Base(p.AbsPath())
+}
+
 func MustAlbum(p Info) string {
 	if r := p.Album(); r != "" {
 		return r
 	}
-	return FallbackAlbum
+
+	// return the dir name for album name
+	return path.Base(path.Dir(p.AbsPath()))
 }
 
 func MustArtist(p Info) string {

tags/taglib/taglib.go

@@ -28,12 +28,13 @@ func (TagLib) Read(absPath string) (tagcommon.Info, error) {
 	defer f.Close()
 	props := f.ReadAudioProperties()
 	raw := f.ReadTags()
-	return &info{raw, props}, nil
+	return &info{raw, props, absPath}, nil
 }
 
 type info struct {
 	raw     map[string][]string
 	props   *audiotags.AudioProperties
+	abspath string
 }
 
 // https://picard-docs.musicbrainz.org/downloads/MusicBrainz_Picard_Tag_Map.html
@@ -60,6 +61,8 @@ func (i *info) ReplayGainAlbumPeak() float32 { return flt(first(find(i.raw, "rep
 func (i *info) Length() int  { return i.props.Length }
 func (i *info) Bitrate() int { return i.props.Bitrate }
 
+func (i *info) AbsPath() string { return i.abspath }
+
 func first[T comparable](is []T) T {
 	var z T
 	for _, i := range is {