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/layout/Sidebar.tsx

@@ -1,3 +1,5 @@
+import { useTranslation } from 'react-i18next'
+
 import { ScrollArea } from '@/components/ui/scroll-area'
 import { cn } from '@/lib/utils'
 import { useBackground } from '@/hooks/use-background'
@@ -20,6 +22,7 @@ export function Sidebar({
   tooltipsEnabled, 
   onMobileMenuClose 
 }: SidebarProps) {
+  const { t } = useTranslation()
   const sidebarBg = useBackground('sidebar')
 
   return (
@@ -60,7 +63,7 @@ export function Sidebar({
                 !sidebarOpen && "lg:mb-1 lg:invisible"
               )}>
                 <h3 className="text-xs font-semibold uppercase tracking-wider text-muted-foreground/60 whitespace-nowrap">
-                  {section.title}
+                  {t(section.title)}
                 </h3>
               </div>