refactor: extract magic numbers to constants

Add constants for sidebar constraints and scroll speed:
- sidebarMinWidth (20)
- sidebarHideWidth (10)
- scrollLines (3)

Also add comment explaining Width(0) for grab line.

Author: pablospe

pkg/ui/mainModel.go

@@ -35,6 +35,13 @@ const (
 
 	// Sidebar resize detection threshold in pixels.
 	sidebarGrabThreshold = 2
+
+	// Sidebar width constraints.
+	sidebarMinWidth  = 20
+	sidebarHideWidth = 10
+
+	// Scroll speed in lines per wheel tick.
+	scrollLines = 3
 )
 
 type mainModel struct {
@@ -265,6 +272,7 @@ func (m mainModel) View() string {
 			BorderForeground(lipgloss.Color("8")).Render(content)
 	} else {
 		// Show a thin grab line when sidebar is hidden.
+		// Width(0) means only the border is rendered (1 char).
 		grabLine := lipgloss.NewStyle().
 			Width(0).
 			Height(m.height - footerHeight - headerHeight).
@@ -505,7 +513,7 @@ func (m mainModel) handleFileTreeClick(msg tea.MouseMsg) (tea.Model, tea.Cmd) {
 }
 
 func (m mainModel) handleScroll(msg tea.MouseMsg) (tea.Model, tea.Cmd) {
-	lines := 3
+	lines := scrollLines
 
 	// Check if scrolling in sidebar (file tree or search results).
 	if zone.Get(zoneFileTree).InBounds(msg) || zone.Get(zoneSearchResults).InBounds(msg) {
@@ -538,15 +546,15 @@ func (m mainModel) handleScroll(msg tea.MouseMsg) (tea.Model, tea.Cmd) {
 
 func (m mainModel) handleSidebarDrag(msg tea.MouseMsg) (tea.Model, tea.Cmd) {
 	// Hide sidebar if dragged below threshold.
-	if msg.X < 10 {
+	if msg.X < sidebarHideWidth {
 		m.isShowingFileTree = false
 		m.draggingSidebar = false
 		cmd := m.diffViewer.SetSize(m.width, m.height-footerHeight-headerHeight)
 		return m, cmd
 	}
 
 	// Clamp to reasonable bounds.
-	minWidth := 20
+	minWidth := sidebarMinWidth
 	maxWidth := m.width / 2
 	newWidth := max(minWidth, min(maxWidth, msg.X))