feat: 添加广告激励下载功能

- 优化useAd Hook支持奖励和关闭回调
- 在预览页面添加看广告下载按钮
- 实现下载图片到本地相册功能
- 添加完整的中文注释和业务逻辑处理

src/pages/index/index.css

@@ -404,4 +404,74 @@
   50% {
     opacity: 1;
   }
+}
+
+/* 下载区域样式 */
+.download-section {
+  position: absolute;
+  bottom: 60rpx;
+  left: 50%;
+  transform: translateX(-50%);
+  width: 100%;
+  padding: 0 40rpx;
+  box-sizing: border-box;
+  z-index: 1001;
+  text-align: center;
+}
+
+/* 下载按钮样式 */
+.download-btn {
+  background: linear-gradient(135deg, #ff6b9d, #ff9a56);
+  border: none;
+  border-radius: 50rpx;
+  padding: 20rpx 40rpx;
+  font-size: 28rpx;
+  font-weight: bold;
+  color: #fff;
+  text-shadow: 0 2px 4px rgba(0, 0, 0, 0.3);
+  box-shadow: 0 8rpx 25rpx rgba(255, 107, 157, 0.4);
+  min-width: 320rpx;
+  transition: all 0.3s ease;
+  position: relative;
+  overflow: hidden;
+}
+
+.download-btn::before {
+  content: "";
+  position: absolute;
+  top: 0;
+  left: -100%;
+  width: 100%;
+  height: 100%;
+  background: linear-gradient(
+    90deg,
+    transparent,
+    rgba(255, 255, 255, 0.3),
+    transparent
+  );
+  transition: left 0.5s;
+}
+
+.download-btn:hover::before {
+  left: 100%;
+}
+
+.download-btn:active {
+  transform: translateY(2rpx);
+  box-shadow: 0 4rpx 15rpx rgba(255, 107, 157, 0.3);
+}
+
+.download-btn:disabled {
+  opacity: 0.6;
+  cursor: not-allowed;
+}
+
+/* 下载提示文字 */
+.download-tip {
+  font-size: 24rpx;
+  color: rgba(255, 255, 255, 0.8);
+  margin-top: 20rpx;
+  text-shadow: 0 1px 2px rgba(0, 0, 0, 0.5);
+  line-height: 1.4;
+  display: block;
 }