Add Semi UI internationalization to the project by wrapping the root
component tree with `LocaleProvider`. A new `SemiLocaleWrapper`
component maps the current `i18next` language code to the corresponding
Semi locale (currently `zh_CN` and `en_GB`) and falls back to Chinese
when no match is found.
Key changes
-----------
1. web/src/index.js
• Import `LocaleProvider`, `useTranslation`, and Semi locale files.
• Introduce `SemiLocaleWrapper` to determine `semiLocale` from
`i18next.language` using a concise prefix-based mapping.
• Wrap `PageLayout` with `SemiLocaleWrapper` inside the existing
`ThemeProvider`.
2. Ensures that all Semi components automatically display the correct
language when the app language is switched via i18next.
BREAKING CHANGE
---------------
Applications embedding this project must now ensure that `i18next`
initialization occurs before React render so that `LocaleProvider`
receives the correct initial language.
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