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/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()
+}