feat(library): remove tracks from the library without touching the file

"Remove from library" deletes the audio_files row and records the path
as excluded, so the next scan does not import it again. Without the
exclusion the operation undoes itself on the next scan, which is worse
than not having it at all; the file on disk is never touched, which is
the promise the confirmation copy will make.

The soft scan compares the number of audio files on disk against the
number of rows, so both walks now skip excluded paths — otherwise the
two counts disagree forever and every launch queues a full scan of the
whole library. A full rescan clears the exclusions, which is the only
way back for a path removed by mistake until there is a UI for it.

backend/library/library.go

@@ -334,7 +334,13 @@ func (l *Library) scanInternal(
 	// --- Pre-walk: count audio files for progress reporting ---
 	emitProgress(mkProgress("counting", 0, 0, 0, 0, 0))
 
-	totalFiles := countAudioFiles(basePath)
+	// Paths the user has removed from the library.  Loaded once per
+	// scan: the walk consults it per file, and the counts above and
+	// below must agree with it or the progress bar and the soft scan
+	// both describe a library that is not the one being built.
+	excluded := l.excludedPathSet(libraryID)
+
+	totalFiles := countAudioFiles(basePath, excluded)
 
 	l.logger.Debug(
 		"pre-walk file count complete",
@@ -423,6 +429,20 @@ func (l *Library) scanInternal(
 					return nil
 				}
 
+				// The user removed this path from the library.  Leaving
+				// it out of existingPaths' LoadAndDelete as well is
+				// deliberate: if a row somehow exists for an excluded
+				// path, orphan cleanup below deletes it, which is the
+				// state the user asked for.
+				if isExcluded(excluded, absoluteFilePath) {
+					l.logger.Debug(
+						"skipping excluded path",
+						"path", absoluteFilePath,
+					)
+
+					return nil
+				}
+
 				// Stat the entry for the staleness comparison below.
 				// This happens before the file is read, so a file
 				// modified mid-scan records the pre-read mtime and is
@@ -1088,21 +1108,27 @@ func (l *Library) pruneOrphanedMetadata() {
 // countAudioFiles performs a fast walk of the library directory,
 // counting only files with supported audio extensions.  No per-file
 // I/O is performed — this reads only directory entries.
-func countAudioFiles(basePath string) int64 {
+func countAudioFiles(basePath string, excluded map[string]struct{}) int64 {
 	var count int64
 
 	_ = fs.WalkDir(
 		os.DirFS(basePath), ".",
-		func(_ string, d fs.DirEntry, err error) error {
+		func(path string, d fs.DirEntry, err error) error {
 			if err != nil || d.IsDir() {
 				return nil
 			}
 
 			ext := filepath.Ext(d.Name())
-			if _, ok := metadata.GetSupportedFileType(ext); ok {
-				count++
+			if _, ok := metadata.GetSupportedFileType(ext); !ok {
+				return nil
 			}
 
+			if isExcluded(excluded, filepath.Join(basePath, path)) {
+				return nil
+			}
+
+			count++
+
 			return nil
 		},
 	)
@@ -1119,10 +1145,17 @@ func countAudioFiles(basePath string) int64 {
 // Unlike countAudioFiles this stats every entry, so it is the more
 // expensive of the two walks.  Only the startup soft scan uses it —
 // the in-scan progress total does not need mtimes.
-func surveyAudioFiles(basePath string) (count, maxModTime int64) {
+//
+// Both walks take the library's excluded paths and skip them, because
+// both answer "how many files would a scan import", not "how many
+// files are there".
+func surveyAudioFiles(
+	basePath string,
+	excluded map[string]struct{},
+) (count, maxModTime int64) {
 	_ = fs.WalkDir(
 		os.DirFS(basePath), ".",
-		func(_ string, d fs.DirEntry, err error) error {
+		func(path string, d fs.DirEntry, err error) error {
 			if err != nil || d.IsDir() {
 				return nil
 			}
@@ -1132,6 +1165,14 @@ func surveyAudioFiles(basePath string) (count, maxModTime int64) {
 				return nil
 			}
 
+			// An excluded path is not a file this scan would import,
+			// so it must not be counted: the soft scan compares this
+			// count against the database's, and a permanent
+			// disagreement queues a full scan on every launch.
+			if isExcluded(excluded, filepath.Join(basePath, path)) {
+				return nil
+			}
+
 			count++
 
 			info, infoErr := d.Info()