refactor(ui): update action panel and rollback button

- Replace `UIUtil.DEFAULT_HGAP` with a fixed value `4` in `actionPanel`.
- Set `isOpaque` to `true` for `actionPanel`.
- Rename "Rollback" button to "Undo" for clarity

core/src/main/kotlin/cc/unitmesh/devti/sketch/ui/patch/SingleFileDiffView.kt

@@ -82,7 +82,8 @@ class SingleFileDiffView(
             })
         }
 
-        val actionPanel = JPanel(HorizontalLayout(UIUtil.DEFAULT_HGAP)).apply {
+        val actionPanel = JPanel(HorizontalLayout(4)).apply {
+            isOpaque = true
             actions.forEach { button ->
                 add(button)
             }
@@ -151,7 +152,7 @@ class SingleFileDiffView(
         val undoManager = UndoManager.getInstance(myProject)
         val fileEditor = FileEditorManager.getInstance(myProject).getSelectedEditor(currentFile)
 
-        val rollback = JButton("Rollback").apply {
+        val rollback = JButton("Undo").apply {
             icon = AllIcons.Actions.Rollback
             toolTipText = AutoDevBundle.message("sketch.patch.action.rollback.tooltip")
             isEnabled = undoManager.isUndoAvailable(fileEditor)