fix(ui): add subtle background fill to user messages

Use theme.Highlight (Catppuccin Surface 1) as a background color on
user message blocks for visual distinction from agent messages.

internal/ui/block_renderer.go

@@ -10,6 +10,7 @@ import (
 type blockRenderer struct {
 	align         *lipgloss.Position
 	borderColor   *color.Color
+	bgColor       *color.Color
 	fullWidth     bool
 	noBorder      bool
 	paddingTop    int
@@ -33,6 +34,14 @@ func WithFullWidth() renderingOption {
 	}
 }
 
+// WithBackground returns a renderingOption that sets a background color
+// for the entire block.
+func WithBackground(c color.Color) renderingOption {
+	return func(br *blockRenderer) {
+		br.bgColor = &c
+	}
+}
+
 // WithNoBorder returns a renderingOption that disables all borders on the
 // block, rendering content with only padding.
 func WithNoBorder() renderingOption {
@@ -145,6 +154,10 @@ func renderContentBlock(content string, containerWidth int, options ...rendering
 		PaddingRight(renderer.paddingRight).
 		Foreground(theme.Text)
 
+	if renderer.bgColor != nil {
+		style = style.Background(*renderer.bgColor)
+	}
+
 	// Border width used for full-width calculation.
 	borderChars := 0
 

internal/ui/messages.go

@@ -109,6 +109,7 @@ func (r *MessageRenderer) RenderUserMessage(content string, timestamp time.Time)
 		r.width,
 		WithAlign(lipgloss.Left),
 		WithBorderColor(theme.Primary),
+		WithBackground(theme.Highlight),
 		WithMarginBottom(1),
 	)