i18n: translate frontend UI strings to English

src/components/Sidebar.tsx

@@ -29,11 +29,11 @@ export default function Sidebar({ user, onLogout }: Props) {
   }, [user])
 
   const links = user ? [
-    { to: '/', icon: '📊', label: '仪表盘' },
+    { to: '/', icon: '📊', label: 'Dashboard' },
     { to: '/issues', icon: '📋', label: 'Issues' },
-    { to: '/projects', icon: '📁', label: '项目' },
-    { to: '/milestones', icon: '🏁', label: '里程碑' },
-    { to: '/notifications', icon: '🔔', label: '通知' + (unreadCount > 0 ? ' (' + unreadCount + ')' : '') },
+    { to: '/projects', icon: '📁', label: 'Projects' },
+    { to: '/milestones', icon: '🏁', label: 'Milestones' },
+    { to: '/notifications', icon: '🔔', label: 'Notifications' + (unreadCount > 0 ? ' (' + unreadCount + ')' : '') },
     { to: '/monitor', icon: '📡', label: 'Monitor' },
   ] : [
     { to: '/monitor', icon: '📡', label: 'Monitor' },
@@ -54,7 +54,7 @@ export default function Sidebar({ user, onLogout }: Props) {
       {user && (
         <div className="sidebar-footer">
           <span>👤 {user.username}</span>
-          <button onClick={onLogout}>退出</button>
+          <button onClick={onLogout}>Logout</button>
         </div>
       )}
     </nav>