fix(formplayer): localize progress bar navigation labels #907

The small adventure

ODE forms can be used in more than one language, sometimes by people collecting
data far from a reliable network connection. The Formplayer already translates
its visible interface into English, Portuguese, and French, but the progress
bar's screen-reader labels are still hard-coded in English.

Let's make those two quiet little chevrons speak the same language as the rest
of the form.

Where to start

• formulus-formplayer/src/components/FormProgressBar.tsx
• formulus-formplayer/src/components/FormProgressBar.test.tsx
• formulus-formplayer/src/i18n/useOdeT.ts
• formulus-formplayer/src/locales/{en,pt,fr}.json
• formulus-formplayer/AGENTS.md

The translation keys already exist:

• nav.previousScreen
• nav.nextScreen

What to change

• Use Formplayer's existing translation hook for the previous and next button
  aria-label values.
• Preserve the current English fallback text.
• Extend the component tests to prove that at least one non-English locale is
  used correctly.
• Keep the visible behavior, navigation callbacks, and disabled states exactly
  as they are today.

Done when

• The two navigation buttons use the active ODE UI locale.
• English, Portuguese, and French continue to use the existing catalog
  entries; no duplicate strings are introduced.
• Tests cover the localized accessible labels and the existing navigation
  tests remain green.
• Lint, formatting, tests, and build pass for Formplayer.

Verify it

From formulus-formplayer/:

pnpm run lint
pnpm run format:check
pnpm test run
pnpm run build

What you will meet along the way

This issue is a compact tour of Formplayer: the React app that renders JSON
Forms inside Formulus and ODE Desktop. It also shows ODE's two localization
layers: ODE-owned interface text and form-author-owned content.

AI tools are welcome, not required

An AI assistant can help trace how FormplayerLocaleContext reaches a component
or suggest a test setup. Please still follow that path yourself and explain in
the pull request why the chosen hook updates when the locale changes. The goal
is a working fix and a new contributor who knows this corner of ODE.

If the test context is unclear, ask in the issue before building a new i18n test
helper. There may already be a tiny pattern worth reusing.