feat(dev): support Vercel serverless API handlers locally in Vite dev server

vite.config.ts

@@ -4,6 +4,14 @@ import path from 'path';
 import {defineConfig, loadEnv} from 'vite';
 import { getBcConfig, getBCToken, findItem, patchItemCpnpNo, fetchAllItems, buildWorkbook } from './bc-runtime.js';
 import * as XLSX from 'xlsx';
+// @ts-ignore
+import dropboxProxyHandler from './api/dropbox-proxy.js';
+// @ts-ignore
+import dropboxSyncHandler from './api/dropbox-sync.js';
+// @ts-ignore
+import backupHandler from './api/backup.js';
+// @ts-ignore
+import usersAdminHandler from './api/users-admin.js';
 
 async function readJsonBody(req: any): Promise<any> {
   return await new Promise((resolve, reject) => {
@@ -35,6 +43,58 @@ export default defineConfig(({mode}) => {
               const url = new URL(req.url || '/', 'http://localhost');
               const config = getBcConfig(env);
 
+              // Populate process.env with loaded env variables for serverless handlers
+              Object.assign(process.env, env);
+
+              // Helper to adapt Node.js req/res to Vercel signature
+              const adaptVercelHandler = async (handler: any) => {
+                (req as any).query = Object.fromEntries(url.searchParams.entries());
+                try {
+                  (req as any).body = await readJsonBody(req);
+                } catch {
+                  (req as any).body = {};
+                }
+                (res as any).status = (code: number) => {
+                  res.statusCode = code;
+                  return res;
+                };
+                (res as any).json = (data: any) => {
+                  res.setHeader('Content-Type', 'application/json');
+                  res.end(JSON.stringify(data));
+                };
+                (res as any).send = (data: any) => {
+                  if (data instanceof Buffer) {
+                    res.end(data);
+                  } else if (typeof data === 'object') {
+                    res.setHeader('Content-Type', 'application/json');
+                    res.end(JSON.stringify(data));
+                  } else {
+                    res.end(data);
+                  }
+                };
+                await handler(req, res);
+              };
+
+              if (url.pathname === '/api/dropbox-proxy') {
+                await adaptVercelHandler(dropboxProxyHandler);
+                return;
+              }
+
+              if (url.pathname === '/api/dropbox-sync') {
+                await adaptVercelHandler(dropboxSyncHandler);
+                return;
+              }
+
+              if (url.pathname === '/api/backup') {
+                await adaptVercelHandler(backupHandler);
+                return;
+              }
+
+              if (url.pathname === '/api/users-admin') {
+                await adaptVercelHandler(usersAdminHandler);
+                return;
+              }
+
               if (url.pathname === '/api/bc-proxy') {
                 if (req.method === 'OPTIONS') {
                   res.statusCode = 204;