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>

docs/TODO.md

@@ -0,0 +1,48 @@
+# TODO — UX / UI improvements
+
+Backlog of usability and interface improvements for the installer TUI. Ordered by
+value-to-effort. Status: ☐ pending · ☑ done.
+
+## High value
+
+- ☑ **1. Back navigation between steps.** Maintain a history stack of *input* steps
+  (forms + IP question + review) and bind `Esc` to go back. Action steps (download,
+  generate, run) are not part of the stack and are not re-enterable via back. Form
+  values are preserved because the `FormStep`s live on the `Model`.
+
+- ☑ **2. Review/summary step before running containers.** Show all collected config
+  (`StepReview`) for confirmation before `StepGenerateFile`/`StepRunDocker`. Natural
+  anchor for "go back and edit a section" via `Esc`.
+
+- ☑ **3. Retry instead of quit on transient errors.** Image pull / container run
+  failures offer `r: tentar novamente` (re-runs just that command) instead of throwing
+  away the whole session. Validation errors (generate file) route back to the relevant
+  form to fix the value.
+
+## Medium value
+
+- ☐ **4. Center the layout (vertically + horizontally).** Use
+  `lipgloss.Place(m.width, m.height, lipgloss.Center, lipgloss.Center, body)` — the
+  Model already tracks `width`/`height`. Caveat: every `viewXxx`/`form.View` currently
+  hardcodes a manual left pad (`strings.Repeat(" ", padding)`); to center cleanly,
+  strip that and wrap the body in one padded/bordered box, then `Place` it. Consider
+  pinning the help footer to the bottom rather than centering it with the body.
+
+- ☐ **5. Inline field validation.** Numeric validation currently happens only at file
+  write time (`WriteConfigFile`), far from input. Wire `textinput.Validate` per field
+  and render a red hint under invalid fields in `form.go`.
+
+- ☐ **6. Step indicator.** Show "Etapa N / Total" or a breadcrumb in the header. Total
+  is dynamic because the vproxy branch adds steps.
+
+## Low value / cosmetic
+
+- ☐ **7. Replace the fake progress bar.** `viewCheckDocker` animates a random-increment
+  bar for an instant `LookPath` check, then waits for a keypress. Use a spinner that
+  resolves immediately (or auto-advance on success).
+
+- ☐ **8. Bordered card + `bubbles/help` footer.** Wrap the body in a rounded
+  `lipgloss` border and render the footer via `bubbles/help` with a proper key map and
+  a `?` toggle. Pairs with #4.
+
+- ☐ **9. Password reveal toggle** (`ctrl+r`) on the login password field.