fix: 优化阿里生图分辨率计算，确保宽高比和分辨率正确配合

- 改进 clamp 逻辑：超出范围时按比例缩放而非直接截断
- 确保在缩放后仍保持正确的宽高比
- 分辨率等级(1K/2K/4K)和宽高比(4:3/16:9等)可以正确组合

lib/ai/providers/alibaba-image.ts

@@ -399,20 +399,45 @@ export function createAlibabaImage({ apiKey, baseURL }: AlibabaImageConfig) {
             };
             
             const [w, h] = ratioMap[ratio] || [1, 1];
+            const aspectValue = w / h;
             
             // 计算宽高，保持总像素接近 base*base
             const totalPixels = base * base;
-            const aspectValue = w / h;
             
             // height = sqrt(totalPixels / aspectValue)
             // width = height * aspectValue
             let height = Math.round(Math.sqrt(totalPixels / aspectValue));
             let width = Math.round(height * aspectValue);
             
-            // 确保宽高在 512-2048 范围内
+            // 阿里云 API 限制：宽高在 512-2048 之间
-            const clamp = (val: number) => Math.max(512, Math.min(2048, val));
+            const MIN_SIZE = 512;
-            width = clamp(width);
+            const MAX_SIZE = 2048;
-            height = clamp(height);
+            
+            // 如果超出范围，按比例缩放，同时保持宽高比
+            if (width > MAX_SIZE) {
+              const scale = MAX_SIZE / width;
+              width = MAX_SIZE;
+              height = Math.round(height * scale);
+            }
+            if (height > MAX_SIZE) {
+              const scale = MAX_SIZE / height;
+              height = MAX_SIZE;
+              width = Math.round(width * scale);
+            }
+            if (width < MIN_SIZE) {
+              const scale = MIN_SIZE / width;
+              width = MIN_SIZE;
+              height = Math.round(height * scale);
+            }
+            if (height < MIN_SIZE) {
+              const scale = MIN_SIZE / height;
+              height = MIN_SIZE;
+              width = Math.round(width * scale);
+            }
+            
+            // 最终 clamp 确保在范围内
+            width = Math.max(MIN_SIZE, Math.min(MAX_SIZE, width));
+            height = Math.max(MIN_SIZE, Math.min(MAX_SIZE, height));
             
             // 确保是偶数（某些图像处理可能需要）
             width = Math.round(width / 2) * 2;