i18n: extend component locales in story (#2563)

## Summary

- upgrade to rust-i18n 4.2.0 and register Story locales with
`extend!(gpui_component)`
- add French Calendar and DatePicker translations as a Story-only
extension
- keep the component’s built-in Italian and Traditional Chinese locales
unchanged
- add French to the Story language menu and cover extension fallback
behavior with a test
- document custom I18n setup, namespace rules, override priority, and
locale selection in English and Simplified Chinese

This uses rust-i18n’s locale extension mechanism to address the
customization requirement discussed in #2548 without adding every
downstream locale to gpui-component itself.

Refs #2548

## Test Plan

- `cargo test -p gpui-component-story`
- `git diff --check`
- `bun run build` *(blocked by the current Bun/Vite Vue runtime:
`crypto.hash is not a function` while compiling the existing
`contributors.md`)*

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Co-authored-by: Codex <codex@openai.com>