feat(ui): 添加基础 UI 组件

- Avatar: 用户头像组件，支持图片和文字头像
- Toggle: 开关切换组件，用于设置项
- AILogo: AI 助手 Logo 组件，品牌标识

src/components/ui/AILogo.tsx

@@ -0,0 +1,26 @@
+'use client';
+
+import { cn } from '@/lib/utils';
+
+interface AILogoProps {
+  size?: number;
+  className?: string;
+}
+
+export function AILogo({ size = 48, className }: AILogoProps) {
+  return (
+    <svg
+      viewBox="0 0 48 48"
+      fill="none"
+      xmlns="http://www.w3.org/2000/svg"
+      width={size}
+      height={size}
+      className={cn('text-[var(--color-primary)]', className)}
+    >
+      <path
+        d="M24 4L26.5 18.5L41 16L29.5 24L41 32L26.5 29.5L24 44L21.5 29.5L7 32L18.5 24L7 16L21.5 18.5L24 4Z"
+        fill="currentColor"
+      />
+    </svg>
+  );
+}

src/components/ui/Avatar.tsx

@@ -0,0 +1,31 @@
+'use client';
+
+import { cn } from '@/lib/utils';
+
+interface AvatarProps {
+  name: string;
+  size?: 'sm' | 'md' | 'lg';
+  className?: string;
+}
+
+export function Avatar({ name, size = 'md', className }: AvatarProps) {
+  const initial = name.charAt(0).toUpperCase();
+
+  const sizeClasses = {
+    sm: 'w-6 h-6 text-xs',
+    md: 'w-8 h-8 text-sm',
+    lg: 'w-10 h-10 text-base',
+  };
+
+  return (
+    <div
+      className={cn(
+        'flex items-center justify-center rounded-full bg-[var(--color-primary)] text-white font-semibold',
+        sizeClasses[size],
+        className
+      )}
+    >
+      {initial}
+    </div>
+  );
+}

src/components/ui/Toggle.tsx

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+'use client';
+
+import { cn } from '@/lib/utils';
+
+interface ToggleProps {
+  checked: boolean;
+  onChange: (checked: boolean) => void;
+  className?: string;
+}
+
+export function Toggle({ checked, onChange, className }: ToggleProps) {
+  return (
+    <button
+      type="button"
+      role="switch"
+      aria-checked={checked}
+      onClick={() => onChange(!checked)}
+      className={cn(
+        'relative w-11 h-6 rounded-full transition-colors duration-150 ease-in-out',
+        checked ? 'bg-[var(--color-primary)]' : 'bg-[var(--color-border)]',
+        className
+      )}
+    >
+      <span
+        className={cn(
+          'absolute top-0.5 left-0.5 w-5 h-5 bg-white rounded-full shadow-sm transition-transform duration-150 ease-in-out',
+          checked && 'translate-x-5'
+        )}
+      />
+    </button>
+  );
+}