fix(ssa): harden spreadsheet upload recognition and guidance

Fix SSA data-context generation for Excel uploads by parsing xlsx/xls via extension-aware paths instead of UTF-8 fallback.
Add on-demand overview rebuild in Agent flow, align xls friendly prompts on frontend/backend, and surface backend upload errors to users.

Made-with: Cursor

frontend-v2/src/modules/ssa/hooks/useAnalysis.ts

@@ -91,7 +91,16 @@ export function useAnalysis(): UseAnalysisReturn {
         body: formData,
       });
 
-      if (!response.ok) throw new Error('上传失败');
+      if (!response.ok) {
+        let errMsg = '上传失败';
+        try {
+          const errData = await response.json();
+          errMsg = errData?.error || errData?.message || errMsg;
+        } catch {
+          // ignore parse error
+        }
+        throw new Error(errMsg);
+      }
       const result = await response.json();
       setUploadProgress(100);
       return result;