style(界面): 优化聊天页面布局和用户菜单显示

- 调整聊天输入区域左内边距以适配侧边栏
- 移除用户菜单中的 plan 信息显示
- 简化用户信息展示

src/app/chat/[id]/page.tsx

@@ -495,7 +495,7 @@ export default function ChatPage({ params }: PageProps) {
             background: `linear-gradient(to top, var(--color-bg-secondary) 0%, var(--color-bg-secondary) 80%, transparent 100%)`
           }}
         >
-          <div className="max-w-[900px] mx-auto px-4 pb-4">
+          <div className="max-w-[900px] mx-auto px-4 pb-4 pl-[60px]">
             {isStreaming && (
               <div className="flex justify-center mb-3">
                 <button

src/components/ui/UserMenu.tsx

@@ -98,9 +98,6 @@ export function UserMenu() {
           <div className="text-sm text-[var(--color-text-primary)] truncate">
             {user.email}
           </div>
-          <div className="text-xs text-[var(--color-text-tertiary)] capitalize">
-            {user.plan} plan
-          </div>
         </div>
         {isOpen ? (
           <ChevronUp size={16} className="text-[var(--color-text-tertiary)] flex-shrink-0" />