fix(storage): fix R2 setup issues (CORS, polyfills, headers)

- Switched to `esm.sh` bundle for `@aws-sdk/client-s3` to fix browser compatibility and `fs.readFile` errors.
- Removed `cf-create-bucket-if-missing` header from `multipartUpload` to prevent 403 Forbidden/CORS failures with standard API tokens.
- Refactored `verifyPublicAccess` to attempt CORS setup even if bucket check fails, and added specific error messaging for CORS issues.
- Updated documentation to include `POST` and `HEAD` in the recommended R2 CORS policy.

js/services/r2Service.js

@@ -419,6 +419,10 @@ class R2Service {
     const verifyContent = "bitvid-verification";
     const publicUrl = buildPublicUrl(baseDomain, verifyKey);
 
+    userLogger.info(
+      `[R2] Verifying access for Bucket: '${bucketName}' in Account: '${truncateMiddle(accountId, 6)}'`
+    );
+
     try {
       // 1. Initialize S3
       const s3 = makeR2Client({ accountId, accessKeyId, secretAccessKey });
@@ -490,7 +494,7 @@ class R2Service {
         errorMessage.includes("NetworkError")
       ) {
         errorMessage +=
-          " This is likely a CORS issue. Please enable CORS in your Cloudflare R2 bucket settings for this domain.";
+          " This is likely a CORS issue. Please enable CORS in your Cloudflare R2 bucket settings. Also verify that the Bucket Name exists and your API Token has 'Object Read & Write' permissions.";
       }
       return { success: false, error: errorMessage };
     }

js/storage/r2-s3.js

@@ -251,6 +251,8 @@ export async function multipartUpload({
     AbortMultipartUploadCommand,
   } = requireAwsSdk();
 
+  console.debug("[R2] Starting multipart upload for:", key);
+
   const createCommand = new CreateMultipartUploadCommand({
     Bucket: bucket,
     Key: key,