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

backend/src/modules/ssa/routes/session.routes.ts

@@ -50,13 +50,27 @@ export default async function sessionRoutes(app: FastifyInstance) {
       if (data) {
         const buffer = await data.toBuffer();
         const filename = data.filename;
+        const ext = filename.split('.').pop()?.toLowerCase() || '';
+
+        if (ext === 'xls') {
+          return reply.status(400).send({
+            error: '系统为了保证表格解析的稳定性，当前仅支持 .xlsx / .csv 格式。请您在本地 Excel/WPS 中打开该文件，选择“另存为 -> Excel 工作簿 (.xlsx)”后再次上传。',
+          });
+        }
+
+        const allowedExt = new Set(['csv', 'xlsx']);
+        if (!allowedExt.has(ext)) {
+          return reply.status(400).send({
+            error: `不支持的文件类型: .${ext || 'unknown'}。当前支持：.csv、.xlsx`,
+          });
+        }
+
         const baseName = filename.replace(/\.(csv|xlsx?)$/i, '') || '数据';
         const now = new Date();
         title = `${baseName} ${now.getMonth() + 1}月${now.getDate()}日`;
         
         // 生成存储 Key（遵循 OSS 目录结构规范）
         const uuid = crypto.randomUUID().replace(/-/g, '').substring(0, 16);
-        const ext = filename.split('.').pop()?.toLowerCase() || 'csv';
         dataOssKey = `tenants/${tenantId}/users/${userId}/ssa/${uuid}.${ext}`;
         
         // 上传到 OSS