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/components/http-warning-banner.tsx

@@ -1,4 +1,5 @@
 import { useState } from 'react'
+import { useTranslation } from 'react-i18next'
 import { AlertTriangle, X } from 'lucide-react'
 import { Button } from '@/components/ui/button'
 
@@ -7,6 +8,7 @@ import { Button } from '@/components/ui/button'
  * 当用户通过 HTTP 访问时显示安全警告
  */
 export function HttpWarningBanner() {
+  const { t } = useTranslation()
   // 直接计算初始状态，避免 effect 中调用 setState
   const isHttp = window.location.protocol === 'http:'
   const hostname = window.location.hostname.toLowerCase()
@@ -35,11 +37,11 @@ export function HttpWarningBanner() {
             <AlertTriangle className="h-5 w-5 text-amber-600 dark:text-amber-500 flex-shrink-0" />
             <div className="flex-1">
               <p className="text-sm font-medium text-amber-900 dark:text-amber-100">
-                <span className="font-semibold">安全警告：</span>
-                您正在使用 <strong>HTTP</strong> 访问 MaiBot WebUI
+                <span className="font-semibold">{t('httpWarning.title')}</span>
+                {t('httpWarning.message')}
               </p>
               <p className="text-xs text-amber-800 dark:text-amber-200 mt-1">
-                如果这是公网服务器，您的数据（包括 Token、聊天记录等）可能在传输过程中被窃取。强烈建议使用 HTTPS 访问或仅在本地网络使用。
+                {t('httpWarning.description')}
               </p>
             </div>
           </div>
@@ -48,7 +50,7 @@ export function HttpWarningBanner() {
             size="icon"
             onClick={handleDismiss}
             className="h-8 w-8 text-amber-700 hover:text-amber-900 dark:text-amber-400 dark:hover:text-amber-200 flex-shrink-0"
-            aria-label="关闭警告"
+            aria-label={t('httpWarning.dismiss')}
           >
             <X className="h-4 w-4" />
           </Button>