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/steps.go

@@ -24,8 +24,24 @@ const (
 	StepDatabaseConfig
 	StepCertConfig
 
+	// Review
+	StepReview
+
 	// Finalizing
 	StepGenerateFile
 	StepRunDocker
 	StepDone
 )
+
+// isInputStep reports whether a step collects user input and therefore participates
+// in back navigation. Action/wait steps (downloads, file generation, container runs)
+// are excluded so that "back" never re-enters a side-effecting step.
+func isInputStep(s step) bool {
+	switch s {
+	case StepDockerLogin, StepIPQuestion, StepWireguardConfig,
+		StepAppConfig, StepServerConfig, StepDatabaseConfig,
+		StepCertConfig, StepReview:
+		return true
+	}
+	return false
+}