fix: 图片上传增加压缩(1024px/JPEG 0.7)，对齐 AgnesAIDesktop 方案

问题: 上传图片后 AI 无法识别，回复没有图像分析工具
根因: 原图 base64 直接发送(5-10MB)，Agnes AI 可能因体积过大拒绝处理

对比 AgnesAIDesktop(可正常工作):
- 上传后用 canvas 压缩: max 1024px, JPEG quality 0.7
- 小图(<500KB且尺寸未超标)保留原图

修复:
- ChatInput 新增 compressImage 函数(来自 AgnesAIDesktop 验证方案)
- processFile 中图片读取后立即压缩再存 preview

src/components/chat/ChatInput.tsx

@@ -69,12 +69,13 @@ export function ChatInput(): React.JSX.Element {
     const attachment: Attachment = { id: `att_${Date.now()}_${Math.random().toString(36).slice(2, 6)}`, file, type };
 
     if (type === 'image') {
-      // 图片转 base64 data URL
-      attachment.preview = await new Promise<string>((resolve) => {
+      // 图片转 base64 data URL → 压缩（限制 1024px, JPEG 0.7）
+      const dataUri = await new Promise<string>((resolve) => {
         const reader = new FileReader();
         reader.onload = () => resolve(reader.result as string);
         reader.readAsDataURL(file);
       });
+      attachment.preview = await compressImage(dataUri, 1024, 0.7);
     } else if (type === 'text') {
       // 文本文件读取内容
       attachment.textContent = await new Promise<string>((resolve) => {
@@ -309,3 +310,39 @@ function AttachmentPreview({ attachment, onRemove }: { attachment: Attachment; o
     </Box>
   );
 }
+
+/**
+ * 压缩图片：限制最大边长，输出 JPEG base64。
+ * 小图(< 500KB 且尺寸未超标)直接返回原图，避免重复编码。
+ *
+ * @see AgnesAIDesktop — 已验证 Agnes AI 可正常处理压缩后的图片
+ */
+function compressImage(
+  dataUri: string,
+  maxSize: number,
+  quality: number,
+): Promise<string> {
+  return new Promise((resolve, reject) => {
+    const img = new Image();
+    img.onload = () => {
+      let { width, height } = img;
+      if (width <= maxSize && height <= maxSize && dataUri.length < 500 * 1024) {
+        resolve(dataUri);
+        return;
+      }
+      if (width > height) {
+        if (width > maxSize) { height = Math.round((height * maxSize) / width); width = maxSize; }
+      } else {
+        if (height > maxSize) { width = Math.round((width * maxSize) / height); height = maxSize; }
+      }
+      const canvas = document.createElement('canvas');
+      canvas.width = width;
+      canvas.height = height;
+      const ctx = canvas.getContext('2d')!;
+      ctx.drawImage(img, 0, 0, width, height);
+      resolve(canvas.toDataURL('image/jpeg', quality));
+    };
+    img.onerror = () => reject(new Error('图片加载失败'));
+    img.src = dataUri;
+  });
+}