Add i18n support with Simplified Chinese translations

[buzhidaoqusm]

Summary

This PR adds internationalization(i18n) support to Delta and includes an initial Simplified Chinese (zh-CN) translation.

Instead of replacing English text directly, this introduces a reusable i18n structure so the app can support additional languages more easily in the future.

What changed

• added i18next and react-i18next
• added a new src/i18n/ module for i18n setup and language definitions
• added locale files for:
  • English (en)
  • Simplified Chinese (zh-CN)
• initialized i18n in the app entry point
• added a language selector in the settings panel
• translated major user-facing UI text, including:
  • scan tabs and scan cards
  • snapshot table and settings panel
  • tree header
  • overview panel
  • history chart states
  • notification UI
  • top bar labels
• updated date formatting to follow the active language instead of using a hardcoded locale

Language behavior

• uses the saved language preference if one exists
• otherwise falls back to the system/browser language when supported
• falls back to English if the detected language is not supported

Notes

• this PR focuses on frontend UI localization
• backend diagnostic/error strings are not fully localized yet
• the current structure is intended to make adding more languages straightforward later

Validation

I verified the changes with:

npm run build
npm run tauri build

Manually tested:
switching languages from the settings panel updates the UI correctly