Design: internationalisation and locale

This issue asks for a design, not an implementation.

Problem

Rake has no translation or locale layer. Apps can't switch language, format dates or numbers for the user's locale, or lay out right-to-left text. GPUI Kit's components already contain some locale handling. If Rake adds its own layer separately, text inside Kit components won't follow the app's language.

Questions to answer

1. What does Kit do today for locale and translated strings, and can Rake use the same source of truth?
2. Which message format to use: Fluent, ICU MessageFormat, or plain keys with arguments?
3. How the current locale is detected on each platform and in the browser, and how a change reaches views. This could work like rake::theme::install.
4. Whether view! should support translated strings without making ordinary text harder to write.
5. Date, number and plural formatting, and how much it adds to Web build size.
6. Right-to-left layout: what GPUI supports today and what's missing.
7. How agents and the CLI see translated labels, given that inspection and CDP queries match on accessible names.

Done when

A design doc is merged that answers these questions, with a small prototype in one example and the follow-up implementation issues filed.

Depends on

Adopting GPUI Kit, so the design matches Kit's locale handling.