refactor: extract changePinModel into its own sub-model

The Change PIN screen was the only screen with its state (pinInput,
pinStage, pinMessage) stored directly on the top-level model. Extract
it into a changePinModel in screen_changepin.go to match the pattern
used by all other screens.

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

internal/shell/banking/model.go

@@ -42,10 +42,8 @@ type model struct {
 	transfer   transferModel
 	history    historyModel
 	messages   messagesModel
-	admin      adminModel
+	admin     adminModel
-	pinInput   string
+	changePin changePinModel
-	pinStage   int // 0=old, 1=new, 2=confirm, 3=done
-	pinMessage string
 }
 
 func newModel(sess *shell.SessionContext, bankName, terminalID, region string) *model {
@@ -130,7 +128,7 @@ func (m *model) View() string {
 	case screenMessages:
 		content = m.messages.View()
 	case screenChangePin:
-		content = m.viewChangePin()
+		content = m.changePin.View()
 	case screenAdmin:
 		content = m.admin.View()
 	}
@@ -182,9 +180,7 @@ func (m *model) updateMenu(msg tea.Msg) (tea.Model, tea.Cmd) {
 			return m, tea.Batch(tea.ClearScreen, logAction(m.sess, "MENU 5", "SECURE MESSAGES"))
 		case "6":
 			m.screen = screenChangePin
-			m.pinInput = ""
+			m.changePin = newChangePinModel()
-			m.pinStage = 0
-			m.pinMessage = ""
 			return m, tea.Batch(tea.ClearScreen, logAction(m.sess, "MENU 6", "CHANGE PIN"))
 		case "7":
 			m.quitting = true
@@ -278,95 +274,19 @@ func (m *model) updateMessages(msg tea.Msg) (tea.Model, tea.Cmd) {
 }
 
 func (m *model) updateChangePin(msg tea.Msg) (tea.Model, tea.Cmd) {
-	keyMsg, ok := msg.(tea.KeyMsg)
+	prevStage := m.changePin.stage
-	if !ok {
+	var cmd tea.Cmd
-		return m, nil
+	m.changePin, cmd = m.changePin.Update(msg)
+
+	// Log successful PIN change.
+	if m.changePin.stage == 3 && prevStage != 3 {
+		cmd = tea.Batch(cmd, logAction(m.sess, "CHANGE PIN", "PIN CHANGED SUCCESSFULLY"))
 	}
 
-	if m.pinStage == 3 {
+	if m.changePin.done {
-		return m, m.goToMenu()
+		return m, tea.Batch(cmd, m.goToMenu())
 	}
-
+	return m, cmd
-	switch keyMsg.Type {
-	case tea.KeyEnter:
-		switch m.pinStage {
-		case 0:
-			if m.pinInput != "" {
-				m.pinStage = 1
-				m.pinInput = ""
-			}
-		case 1:
-			if len(m.pinInput) >= 4 {
-				m.pinMessage = m.pinInput
-				m.pinStage = 2
-				m.pinInput = ""
-			}
-		case 2:
-			if m.pinInput == m.pinMessage {
-				m.pinStage = 3
-				return m, logAction(m.sess, "CHANGE PIN", "PIN CHANGED SUCCESSFULLY")
-			}
-			m.pinInput = ""
-			m.pinMessage = ""
-			m.pinStage = 1
-		}
-	case tea.KeyEscape:
-		return m, m.goToMenu()
-	case tea.KeyBackspace:
-		if len(m.pinInput) > 0 {
-			m.pinInput = m.pinInput[:len(m.pinInput)-1]
-		}
-	default:
-		ch := keyMsg.String()
-		if len(ch) == 1 && ch[0] >= 32 && ch[0] < 127 && len(m.pinInput) < 12 {
-			m.pinInput += ch
-		}
-	}
-	return m, nil
-}
-
-func (m *model) viewChangePin() string {
-	var b strings.Builder
-
-	b.WriteString("\n")
-	b.WriteString(centerText("CHANGE PIN"))
-	b.WriteString("\n\n")
-	b.WriteString(thinDivider())
-	b.WriteString("\n\n")
-
-	if m.pinStage == 3 {
-		b.WriteString(titleStyle.Render("  PIN CHANGED SUCCESSFULLY"))
-		b.WriteString("\n\n")
-		b.WriteString(baseStyle.Render("  YOUR NEW PIN IS NOW ACTIVE."))
-		b.WriteString("\n")
-		b.WriteString(baseStyle.Render("  PLEASE USE YOUR NEW PIN FOR ALL FUTURE TRANSACTIONS."))
-		b.WriteString("\n\n")
-		b.WriteString(dimStyle.Render("  PRESS ANY KEY TO RETURN TO MAIN MENU"))
-	} else {
-		prompts := []string{"  CURRENT PIN:  ", "  NEW PIN:      ", "  CONFIRM PIN:  "}
-		for i := 0; i < m.pinStage; i++ {
-			b.WriteString(baseStyle.Render(prompts[i]))
-			b.WriteString(baseStyle.Render(strings.Repeat("*", 4)))
-			b.WriteString("\n")
-		}
-		if m.pinStage < 3 {
-			b.WriteString(titleStyle.Render(prompts[m.pinStage]))
-			masked := strings.Repeat("*", len(m.pinInput))
-			b.WriteString(inputStyle.Render(masked))
-			b.WriteString(inputStyle.Render("_"))
-			b.WriteString("\n")
-		}
-		b.WriteString("\n")
-		if m.pinStage == 1 {
-			b.WriteString(dimStyle.Render("  PIN MUST BE AT LEAST 4 CHARACTERS"))
-			b.WriteString("\n")
-		}
-		b.WriteString("\n")
-		b.WriteString(dimStyle.Render("  PRESS ESC TO RETURN TO MAIN MENU"))
-	}
-	b.WriteString("\n")
-
-	return b.String()
 }
 
 func (m *model) updateAdmin(msg tea.Msg) (tea.Model, tea.Cmd) {

internal/shell/banking/screen_changepin.go

@@ -0,0 +1,111 @@
+package banking
+
+import (
+	"strings"
+
+	tea "github.com/charmbracelet/bubbletea"
+)
+
+type changePinModel struct {
+	input   string
+	stage   int // 0=old, 1=new, 2=confirm, 3=done
+	newPin  string
+	done    bool
+}
+
+func newChangePinModel() changePinModel {
+	return changePinModel{}
+}
+
+func (m changePinModel) Update(msg tea.Msg) (changePinModel, tea.Cmd) {
+	keyMsg, ok := msg.(tea.KeyMsg)
+	if !ok {
+		return m, nil
+	}
+
+	if m.stage == 3 {
+		m.done = true
+		return m, nil
+	}
+
+	switch keyMsg.Type {
+	case tea.KeyEnter:
+		switch m.stage {
+		case 0:
+			if m.input != "" {
+				m.stage = 1
+				m.input = ""
+			}
+		case 1:
+			if len(m.input) >= 4 {
+				m.newPin = m.input
+				m.stage = 2
+				m.input = ""
+			}
+		case 2:
+			if m.input == m.newPin {
+				m.stage = 3
+			} else {
+				m.input = ""
+				m.newPin = ""
+				m.stage = 1
+			}
+		}
+	case tea.KeyEscape:
+		m.done = true
+	case tea.KeyBackspace:
+		if len(m.input) > 0 {
+			m.input = m.input[:len(m.input)-1]
+		}
+	default:
+		ch := keyMsg.String()
+		if len(ch) == 1 && ch[0] >= 32 && ch[0] < 127 && len(m.input) < 12 {
+			m.input += ch
+		}
+	}
+	return m, nil
+}
+
+func (m changePinModel) View() string {
+	var b strings.Builder
+
+	b.WriteString("\n")
+	b.WriteString(centerText("CHANGE PIN"))
+	b.WriteString("\n\n")
+	b.WriteString(thinDivider())
+	b.WriteString("\n\n")
+
+	if m.stage == 3 {
+		b.WriteString(titleStyle.Render("  PIN CHANGED SUCCESSFULLY"))
+		b.WriteString("\n\n")
+		b.WriteString(baseStyle.Render("  YOUR NEW PIN IS NOW ACTIVE."))
+		b.WriteString("\n")
+		b.WriteString(baseStyle.Render("  PLEASE USE YOUR NEW PIN FOR ALL FUTURE TRANSACTIONS."))
+		b.WriteString("\n\n")
+		b.WriteString(dimStyle.Render("  PRESS ANY KEY TO RETURN TO MAIN MENU"))
+	} else {
+		prompts := []string{"  CURRENT PIN:  ", "  NEW PIN:      ", "  CONFIRM PIN:  "}
+		for i := 0; i < m.stage; i++ {
+			b.WriteString(baseStyle.Render(prompts[i]))
+			b.WriteString(baseStyle.Render(strings.Repeat("*", 4)))
+			b.WriteString("\n")
+		}
+		if m.stage < 3 {
+			b.WriteString(titleStyle.Render(prompts[m.stage]))
+			masked := strings.Repeat("*", len(m.input))
+			b.WriteString(inputStyle.Render(masked))
+			b.WriteString(inputStyle.Render("_"))
+			b.WriteString("\n")
+		}
+		b.WriteString("\n")
+		if m.stage == 1 {
+			b.WriteString(dimStyle.Render("  PIN MUST BE AT LEAST 4 CHARACTERS"))
+			b.WriteString("\n")
+		}
+		b.WriteString("\n")
+		b.WriteString(dimStyle.Render("  PRESS ESC TO RETURN TO MAIN MENU"))
+	}
+	b.WriteString("\n")
+
+	return b.String()
+}