feat(dashboard): add i18n support with zh/en/ja/ko locales

- Add react-i18next + i18next + i18next-browser-languagedetector
- Create i18n config (singleton import) with zh/en/ja/ko JSON locale files
- Add language switcher Globe dropdown in Header topbar
- Replace all hardcoded Chinese strings in:
  - Layout (Header, Sidebar, NavItem, Layout, constants)
  - Settings (index, AppearanceTab, SecurityTab, OtherTab, AboutTab)
  - Auth page (auth.tsx)
  - Search dialog (searchItems via useMemo + t())
  - Restart overlay (getStatusConfig accepts t param)
  - Error boundary (ErrorFallback, ErrorDetails function components)
  - HTTP warning banner
- localStorage key: maibot-locale
- Compatible with Electron

dashboard/src/main.tsx

@@ -2,6 +2,7 @@ import { StrictMode, useEffect, useState } from 'react'
 import { createRoot } from 'react-dom/client'
 import { RouterProvider } from '@tanstack/react-router'
 import './index.css'
+import './i18n'
 import { router } from './router'
 import { AssetStoreProvider } from './components/asset-provider'
 import { ThemeProvider } from './components/theme-provider'