feat: improve some translation options

[isolcat]

Summary

This pull request addresses issue #1971 by adding configurable translation options to the theme-default package in Rspress. The changes enable dynamic translation for the following components:

• SidebarMenu/index.tsx: Replaced hardcoded "Menu" with t('menuText').
• CopyCodeButton.tsx: Replaced hardcoded "Copy code" with t('copyCode').
• code/index.tsx: Replaced hardcoded "Toggle code wrap" with t('toggleCodeWrap').

To support these translations, I added new entries (menuText, copyCode, toggleCodeWrap) to i18n.json with default English and Chinese values. The useI18n hook is utilized to dynamically fetch translations

Note:I personally believe the search box width adjustment (to 960px with full-screen below 640px) is not necessary, as the current design appears sufficient

Related Issue

• Issue #1971

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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