Fix export error in R2 storage module and refine upload workflow

- Exported `ensureBucketExists` from `js/storage/r2-s3.js` to resolve a module import error in `js/services/r2Service.js`.
- Verified the fix via code read.
- Confirmed syntax correctness of `r2-s3.js` modifications.

js/storage/r2-s3.js

@@ -129,7 +129,7 @@ export function makeR2Client({ accountId, accessKeyId, secretAccessKey }) {
   });
 }
 
-async function ensureBucketExists({ s3, bucket }) {
+export async function ensureBucketExists({ s3, bucket }) {
   const { CreateBucketCommand, HeadBucketCommand } = requireAwsSdk();
 
   try {