feat: add script to update existing entry in all language files and update README

[zxdong262]

This PR adds a new script bin/update-entry.js that allows updating an existing key/value in all language files for a given namespace. It also updates the README with instructions for using the new script.

• Adds bin/update-entry.js to update existing entries by key in a namespace
• Updates README.md with usage documentation for the new script

Usage examples and error handling are shown in the updated README section.

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This helps maintain consistency across all translations and makes it easier to update UI strings in bulk.