feat(17-02): implement save flow, cover art editing, and error handling

- Wire saveEdit() to WriteTrackTagsByPath with diff-only TagChanges map
- Add cover art selection via ImageFilePicker with instant blob preview
- Add cover art removal (× button) with clearCoverArt state
- Show saving indicator and disable buttons during save
- Display errors inline in action bar; edit mode stays active on error
- Add ReadFile Go method on FrontendUtil to read cover art bytes
- Add Wails binding for ReadFile
- Clean up all edit state (editValues, pendingCoverArt, errorMessage) on close/cancel

backend/frontendutil/frontendutil.go

@@ -4,6 +4,7 @@ package frontendutil
 import (
 	"context"
 	"fmt"
+	"os"
 
 	"github.com/wailsapp/wails/v2/pkg/runtime"
 )
@@ -90,3 +91,15 @@ func (fe *FrontendUtil) ImageFilePicker() (string, error) {
 
 	return file, nil
 }
+
+// ReadFile reads a file from disk and returns its contents.
+// Used by the frontend to read cover art image files selected
+// via ImageFilePicker.
+func (fe *FrontendUtil) ReadFile(path string) ([]byte, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("read file %q: %w", path, err)
+	}
+
+	return data, nil
+}