Add chat history scrolling via PageUp/PageDown and ctrl+u/ctrl+d

Configure viewport keymap to use page-based scroll keys only, leaving
up/down arrows free for textarea cursor movement. Viewport is now
initialized once with the custom keymap and resized (not recreated)
on window resize. Key messages are routed to both viewport and textarea.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

main.go

@@ -173,8 +173,19 @@ func main() {
 	ta.KeyMap.InsertNewline = key.NewBinding(key.WithKeys("ctrl+j"))
 	ta.Focus()
 
+	vp := viewport.New(0, 0)
+	vp.KeyMap = viewport.KeyMap{
+		PageDown:     key.NewBinding(key.WithKeys("pgdown")),
+		PageUp:       key.NewBinding(key.WithKeys("pgup")),
+		HalfPageDown: key.NewBinding(key.WithKeys("ctrl+d")),
+		HalfPageUp:   key.NewBinding(key.WithKeys("ctrl+u")),
+		Down:         key.NewBinding(key.WithKeys()),
+		Up:           key.NewBinding(key.WithKeys()),
+	}
+
 	p := tea.NewProgram(model{
 		textarea: ta,
+		viewport: vp,
 		loopcfg:  loopcfg,
 		hooks:    hooks,
 	})
@@ -204,15 +215,11 @@ func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 
 	switch msg := msg.(type) {
 	case tea.WindowSizeMsg:
-		if !m.ready {
-			m.viewport = viewport.New(msg.Width, msg.Height-5)
-			m.ready = true
-		} else {
-			m.viewport.Width = msg.Width
-			m.viewport.Height = msg.Height - 5
-		}
+		m.viewport.Width = msg.Width
+		m.viewport.Height = msg.Height - 5
 		m.textarea.SetWidth(msg.Width)
 		m.viewport.SetContent(m.renderDisplay())
+		m.ready = true
 
 	case responseMsg:
 		m.display = msg.display
@@ -243,8 +250,10 @@ func (m model) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
 		}
 	}
 
+	var vpCmd tea.Cmd
+	m.viewport, vpCmd = m.viewport.Update(msg)
 	m.textarea, cmd = m.textarea.Update(msg)
-	return m, cmd
+	return m, tea.Batch(cmd, vpCmd)
 }
 
 func (m model) runLoop(input string) tea.Cmd {