fix(library): support SACD artwork fallback and image sidecars

Refresh missing artwork for unchanged SACD images when a folder cover is added after scanning. Enable image-specific metadata and cover sidecars in the Qt editor while preserving physical track numbers and never writing the ISO.

Validated with SACD regressions, the real Heathen image, 13 Qt editor tests, QML audits and the combined release build.

cx

crates/qbz-qt/qml/views/local/LocalEphemeralPane.qml

@@ -246,7 +246,7 @@ Item {
                         }
                         QbzCircleAction {
                             id: editFolderAlbumBtn
-                            visible: !QbzLocal.localSessionIsDisc
+                            visible: !QbzLocal.localEphemeralIsCd
                                 && root.allAlbums.length === 1
                                 && !QbzLocal.localEphemeralLoading
                             name: "pen-line"
@@ -535,7 +535,7 @@ Item {
                                 spacing: 8
                                 QbzCircleAction {
                                     id: editBlockBtn
-                                    visible: !QbzLocal.localSessionIsDisc
+                                    visible: !QbzLocal.localEphemeralIsCd
                                     name: "pen-line"
                                     onClicked: QbzLocal.ephemeralEditTags(
                                         block.modelData.groupKey)