[storage] fix B2 / generic-S3 uploads ("S3 endpoint is required") — prepareUpload contract

Uploading to a Backblaze B2 (or any generic-S3) bucket failed with "S3 endpoint is
required." even though the connection tested fine.

Root cause: the two upload services have different prepareUpload contracts —
r2Service.prepareUpload(npub, { credentials }) vs s3UploadService.prepareUpload(settings).
MediaUploader calls both the r2 way, so for S3/B2 the npub STRING became the settings →
validateS3Connection saw no endpoint. Compounding it, a storageService.getConnection()
result keeps bucket/endpoint/region under `.meta`, which validateS3Connection reads at
the top level. (R2 worked because its service already uses the (npub, {credentials})
contract.)

Fix:
- s3UploadService.prepareUpload now accepts (npub, { credentials }) AND a direct settings
  object (back-compat), flattening meta → top-level via flattenS3Settings.
- The storage Test now derives/passes the B2 public URL (else the prefix), so it stops
  falsely warning "Public Bucket URL is missing. Uploads will fail."

Tests: tests/services/s3-upload-prepare-contract.test.mjs (3 — meta flatten, npub ignored
when credentials given, direct-settings back-compat). Build + lint clean; upload/storage
suites 46/46.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

js/ui/profileModal/ProfileStorageController.js

@@ -4,6 +4,7 @@ import {
   prepareS3Connection,
   getCorsOrigins,
   deriveB2Endpoint,
+  derivePublicBaseUrl,
 } from "../../services/s3Service.js";
 import { getActiveSigner } from "../../nostr/client.js";
 import { DEFAULT_NIP07_ENCRYPTION_METHODS } from "../../nostr/nip07Permissions.js";
@@ -952,6 +953,11 @@ export class ProfileStorageController {
     } else {
       config.endpoint = endpointOrAccount;
       config.forcePathStyle = forcePathStyle;
+      // Explicit Public Access URL, else derive it (so the test doesn't falsely warn).
+      config.publicBaseUrl =
+        publicBaseUrl ||
+        derivePublicBaseUrl({ endpoint: endpointOrAccount, bucket, forcePathStyle });
+      config.baseDomain = config.publicBaseUrl;
     }
 
     try {