feat: add back navigation, review step, and error retry to TUI

Implements three UX improvements (TODO #1–#3):

- Back navigation: Esc returns to the previous input step. The Model keeps
  a history stack of input steps (advance/goBack helpers); action/wait
  steps are excluded via isInputStep so back never re-enters a
  side-effecting step. Form values are preserved.
- Review step (StepReview): shows all collected config for confirmation
  before any file is written; Enter installs, Esc edits.
- Retry vs. fix: transient failures (image pull, container run) offer
  "r: tentar novamente" instead of quitting; validation failures route
  back to the relevant form to correct the value.

Adds nav_test.go for the history/navigation logic and updates
README/CLAUDE/docs (incl. docs/TODO.md tracking the remaining ideas).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

internal/tui/model.go

@@ -12,6 +12,7 @@ import (
 
 type Model struct {
 	currentStep step
+	history     []step // stack of input steps visited, for back navigation
 	cursor      int
 	width       int
 	height      int
@@ -263,3 +264,31 @@ func InitialModel() Model {
 func (m Model) Init() tea.Cmd {
 	return tea.Batch(CheckDockerCmd(), TickCmd(), m.spinner.Tick)
 }
+
+// advance moves to the next step, recording the current step on the history stack when
+// it is an input step so that Esc can return to it. Consecutive duplicates are skipped.
+func (m *Model) advance(next step) {
+	if isInputStep(m.currentStep) {
+		if n := len(m.history); n == 0 || m.history[n-1] != m.currentStep {
+			m.history = append(m.history, m.currentStep)
+		}
+	}
+	m.currentStep = next
+}
+
+// goBack returns to the previous input step on the history stack, if any. It reports
+// whether the step changed.
+func (m *Model) goBack() bool {
+	if len(m.history) == 0 {
+		return false
+	}
+	last := m.history[len(m.history)-1]
+	m.history = m.history[:len(m.history)-1]
+	m.currentStep = last
+	return true
+}
+
+// canGoBack reports whether Esc would return to a previous step from the current one.
+func (m Model) canGoBack() bool {
+	return isInputStep(m.currentStep) && len(m.history) > 0
+}