📝 docs(mini-program): add placeholder specification and adjust section numbering

- add 2.2 placeholder specification section with Unsplash usage guidelines
- include placeholder URL format, usage examples, keyword classification and implementation code
- adjust font specification from 2.2 to 2.3
- adjust spacing specification from 2.3 to 2.4

.roo/rules/16-mini-program-ui.md

@@ -36,7 +36,61 @@
   - 卡片背景：#ffffff
   - 分割線：#e8e8e8
 
-### 2.2 字体规范
+### 2.2 占位图规范
+
+#### Unsplash 占位图使用
+- **默认占位图**：使用 Unsplash 提供的免费高质量图片
+- **图片尺寸**：根据组件需求选择合适的尺寸
+- **格式**：`https://source.unsplash.com/{width}x{height}/?{关键词}`
+
+#### 使用示例
+- **头像占位图**：`https://source.unsplash.com/200x200/?portrait`
+- **封面占位图**：`https://source.unsplash.com/400x300/?nature`
+- **背景占位图**：`https://source.unsplash.com/800x600/?landscape`
+- **商品图片**：`https://source.unsplash.com/300x300/?product`
+
+#### 关键词分类
+- **人物**：portrait, person, people, man, woman
+- **自然**：nature, landscape, mountain, forest, ocean
+- **城市**：city, building, architecture, street
+- **抽象**：abstract, pattern, texture, gradient
+- **技术**：technology, computer, phone, digital
+- **食物**：food, coffee, fruit, restaurant
+- **商业**：business, office, workspace, meeting
+
+#### 实现代码示例
+```typescript
+// 头像组件
+const AvatarPlaceholder = ({ size = 200 }) => {
+  const placeholderUrl = `https://source.unsplash.com/${size}x${size}/?portrait`;
+  return <Image src={placeholderUrl} mode="aspectFill" />;
+};
+
+// 封面组件
+const CoverPlaceholder = ({ width = 400, height = 300, keyword = 'nature' }) => {
+  const placeholderUrl = `https://source.unsplash.com/${width}x${height}/?${keyword}`;
+  return <Image src={placeholderUrl} mode="aspectFill" />;
+};
+
+// 带错误处理的占位图
+const SmartImage = ({ src, placeholder, ...props }) => {
+  const [imageSrc, setImageSrc] = useState(src || placeholder);
+  
+  const handleError = () => {
+    setImageSrc(placeholder);
+  };
+  
+  return (
+    <Image 
+      src={imageSrc} 
+      onError={handleError}
+      {...props}
+    />
+  );
+};
+```
+
+### 2.3 字体规范
 
 #### 字体家族
 - iOS：-apple-system-font, Helvetica Neue
@@ -53,7 +107,7 @@
 - 正文：400（常规）
 - 强调：500（中等）
 
-### 2.3 间距规范
+### 2.4 间距规范
 
 #### 基础间距
 - 8rpx（4px）