fix(12-02): wait for scan to stop before library removal, surface errors in UI

RemoveLibrary now polls until the cancelled scan goroutine finishes
before proceeding with the removal transaction. Also show removal
errors as toast messages instead of only logging to console, and
explicitly reload library list after successful removal.

backend/library/crud.go

@@ -7,6 +7,7 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
+	"time"
 
 	"github.com/wailsapp/wails/v2/pkg/runtime"
 
@@ -200,8 +201,9 @@ func (l *Library) GetRemovalImpact(libraryID int64) (*RemovalImpact, error) {
 // performing orphan cleanup, phantom metadata conversion, FTS5
 // rebuild, and queue compaction. Returns a summary of what was removed.
 func (l *Library) RemoveLibrary(id int64) (*RemovalSummary, error) {
-	// 1. Cancel active scan for this library.
+	// 1. Cancel active scan for this library and wait for it to stop.
 	l.cancelLibraryScan(id)
+	l.waitForScanIdle(id)
 
 	// 2. Stop playback if the currently-playing track belongs to this library.
 	if l.currentTrackBelongsToLibrary(id) {
@@ -465,6 +467,26 @@ func (l *Library) RemoveLibrary(id int64) (*RemovalSummary, error) {
 	return summary, nil
 }
 
+// waitForScanIdle polls until the specified library is no longer the
+// active scan target. Called after cancelLibraryScan to ensure the
+// scan goroutine has finished before proceeding with removal.
+func (l *Library) waitForScanIdle(id int64) {
+	for range 100 { // up to ~5 seconds
+		l.mu.Lock()
+		active := l.currentScanLibraryID == id
+		l.mu.Unlock()
+
+		if !active {
+			return
+		}
+
+		time.Sleep(50 * time.Millisecond)
+	}
+
+	l.logger.Warn("timed out waiting for scan to stop",
+		"libraryID", id)
+}
+
 // cancelLibraryScan cancels an active scan for the specified library
 // and removes it from the scan queue.
 func (l *Library) cancelLibraryScan(id int64) {