docs: Localize "Appearance" label in mobile navigation for i18n support

### Description On smaller screen widths (responsive view), the dark mode toggle label always displays "Appearance" regardless of the selected language. This should be localized to match the user's selected language. ### Current Behavior The "Appearance" text in the mobile navigation menu remains in English for all language settings (Korean, Japanese, Chinese). ### Expected Behavior The label should display the appropriate translation based on the selected locale: - English: `Appearance` - Korean (한국어): `테마` - Japanese (日本語): `テーマ` - Chinese (简体中文): `主題` ### Proposed Solution Add the `darkModeSwitchLabel` property to each locale's `themeConfig` in the VitePress configuration files: - [docs/.vitepress/en.mts](https://github.com/kss2002/es-toolkit/blob/main/docs/.vitepress/en.mts) - [docs/.vitepress/ko.mts](https://github.com/kss2002/es-toolkit/blob/main/docs/.vitepress/ko.mts) - [docs/.vitepress/ja.mts](https://github.com/kss2002/es-toolkit/blob/main/docs/.vitepress/ja.mts) - [docs/.vitepress/zh_hans.mts](https://github.com/kss2002/es-toolkit/blob/main/docs/.vitepress/zh_hans.mts) ### Environment - VitePress documentation site - Responsive/mobile view ### img <img width="554" height="265" alt="Image" src="https://github.com/user-attachments/assets/e59a5bd5-a10d-49e7-9198-cebc822c73da" />