fix(sync): harden Dropbox loading

vite.config.ts

@@ -31,6 +31,7 @@ async function readJsonBody(req: any): Promise<any> {
 
 export default defineConfig(({mode}) => {
   const env = loadEnv(mode, '.', '');
+  const prodEnv = loadEnv('production', '.', '');
   return {
     plugins: [
       react(),
@@ -44,7 +45,7 @@ export default defineConfig(({mode}) => {
               const config = getBcConfig(env);
 
               // Populate process.env with loaded env variables for serverless handlers
-              Object.assign(process.env, env);
+              Object.assign(process.env, prodEnv, env);
 
               // Helper to adapt Node.js req/res to Vercel signature
               const adaptVercelHandler = async (handler: any) => {