fix(sketch): improve diff sketch UI styling and borders

Add border to wrapper panel and update toggle button styling by removing border painting, setting content area to transparent, and adjusting size from 20px to 24px with empty insets.

Author: phodal

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

@@ -35,6 +35,8 @@ import com.intellij.ui.components.panels.HorizontalLayout
 import com.intellij.ui.components.panels.VerticalLayout
 import com.intellij.util.LocalTimeCounter
 import com.intellij.util.concurrency.annotations.RequiresWriteLock
+import com.intellij.util.ui.JBUI
+import com.intellij.util.ui.UIUtil
 import java.awt.BorderLayout
 import java.awt.Dimension
 import java.awt.event.MouseAdapter
@@ -212,6 +214,7 @@ class SingleFileDiffSketch(
         wrapperPanel.preferredSize = Dimension(wrapperPanel.preferredSize.width, maxOf(200, minHeight))
         wrapperPanel.maximumSize = Dimension(Int.MAX_VALUE, maxOf(200, minHeight))
 
+        wrapperPanel.border = JBUI.Borders.customLine(UIUtil.getBoundsColor(), 1, 1, 1, 1)
         return wrapperPanel
     }
 
@@ -268,7 +271,10 @@ class SingleFileDiffSketch(
 
         val toggleButton = JButton().apply {
             icon = AllIcons.Actions.Diff
-            preferredSize = Dimension(20, 20)
+            preferredSize = Dimension(24, 24)
+            margin = JBUI.emptyInsets()
+            isBorderPainted = false
+            isContentAreaFilled = false
             isOpaque = false
             toolTipText = AutoDevBundle.message("sketch.terminal.show.hide")
             addActionListener {