Fix Merge module file list to use full vertical space

Issue: File list only used half the vertical space, wasting screen real estate.

Changed left panel from VBox to Border layout:
- Top: "Clips to Merge" label and Add/Clear buttons (fixed size)
- Center: File list scroll area (expands to fill remaining space)

The border layout gives the scroll area priority to expand vertically,
maximizing the visible file list area. This is especially important
when merging many clips.

main.go

@@ -1807,11 +1807,18 @@ func (s *appState) showMergeView() {
 	}
 
 	listScroll := container.NewVScroll(listBox)
-	listScroll.SetMinSize(fyne.NewSize(400, 300))
 
-	left := container.NewVBox(
+	// Use border layout so the list expands to fill available vertical space
+	leftTop := container.NewVBox(
 		widget.NewLabelWithStyle("Clips to Merge", fyne.TextAlignLeading, fyne.TextStyle{Bold: true}),
 		container.NewHBox(addBtn, clearBtn),
+	)
+
+	left := container.NewBorder(
+		leftTop,  // top
+		nil,      // bottom
+		nil,      // left
+		nil,      // right
 		ui.NewDroppable(listScroll, func(items []fyne.URI) {
 			var paths []string
 			for _, uri := range items {