fix(tui): reserve stable top row for copy icon to prevent layout shift

Always render the top row in assistant messages so that hovering to
reveal the copy icon does not cause a 1-line layout shift. When not
hovered, the row is filled with spaces (invisible); on hover, the
copy icon appears in the same reserved space.

This fixes the layout shift introduced by 80bccc8 without
reintroducing the spurious blank-line artifact it originally fixed.

Assisted-By: docker-agent

Author: dgageot

pkg/tui/components/message/message.go

@@ -145,22 +145,19 @@ func (mv *messageModel) Render(width int) string {
 			prefix = mv.senderPrefix(msg.Sender)
 		}
 
-		// Show copy icon in the top-right corner when hovered or selected.
-		// AssistantMessageStyle has PaddingTop=0 (unlike UserMessageStyle which has
-		// PaddingTop=1), so we cannot unconditionally prepend topRow+"\n" — doing so
-		// would add a spurious blank line to every message in the default state.
-		// Accept the 1-line layout shift on hover; it is less disruptive than the
-		// blank-line artifact that affects all messages at all times.
+		// Always reserve a top row to avoid layout shifts when the copy icon
+		// appears on hover. When not hovered, the row is filled with spaces
+		// (invisible). AssistantMessageStyle has PaddingTop=0, so this extra
+		// row acts as a stable spacer.
+		innerWidth := width - messageStyle.GetHorizontalFrameSize()
+		topRow := strings.Repeat(" ", innerWidth)
 		if mv.hovered || mv.selected {
-			innerWidth := width - messageStyle.GetHorizontalFrameSize()
 			copyIcon := styles.MutedStyle.Render(types.AssistantMessageCopyLabel)
 			iconWidth := ansi.StringWidth(types.AssistantMessageCopyLabel)
 			padding := max(innerWidth-iconWidth, 0)
-			topRow := strings.Repeat(" ", padding) + copyIcon
-			noTopPaddingStyle := messageStyle.PaddingTop(0)
-			return prefix + noTopPaddingStyle.Width(width).Render(topRow+"\n"+rendered)
+			topRow = strings.Repeat(" ", padding) + copyIcon
 		}
-		return prefix + messageStyle.Render(rendered)
+		return prefix + messageStyle.Width(width).Render(topRow+"\n"+rendered)
 	case types.MessageTypeShellOutput:
 		if rendered, err := markdown.NewRenderer(width).Render(fmt.Sprintf("```console\n%s\n```", msg.Content)); err == nil {
 			return rendered