feat(web): RegisterPage avatar uses transferStore + waitForTransfer helper

Replaces the legacy /api/uploads call in RegisterPage with the tus-based
transferStore path. Extends Transfer to persist the server-assigned
filename (not just attachmentId) since downstream consumers store
attachment.filename on the user/space record.

- transferStore: rename setAttachmentId -> setAttachmentRef(id, attachmentId, filename)
  and add attachmentFilename field to Transfer
- both startUpload + resumeUpload onSuccess paths now record filename
- new utils/waitForTransfer.ts: waitForTransferAttachment(transferId)
  returns {attachmentId, filename}, with immediate-terminal handling
- RegisterPage: silent (tray:false) upload via transferStore, awaits
  the helper, passes the server filename to api.users.update

packages/web/src/utils/waitForTransfer.ts

@@ -0,0 +1,38 @@
+import { useTransferStore } from '../stores/transferStore';
+
+export interface TransferAttachmentRef {
+  attachmentId: string;
+  filename: string;
+}
+
+/**
+ * Wait for a transferStore upload to reach a terminal state.
+ * Resolves with the server-assigned attachmentId + filename on success.
+ * Rejects on failure or abort.
+ *
+ * Handles the already-terminal case synchronously (resolves/rejects immediately
+ * without subscribing) and unsubscribes after the first terminal observation.
+ */
+export function waitForTransferAttachment(transferId: string): Promise<TransferAttachmentRef> {
+  return new Promise<TransferAttachmentRef>((resolve, reject) => {
+    const check = (): boolean => {
+      const t = useTransferStore.getState().transfers.get(transferId);
+      if (!t) return false;
+      if (t.state === 'completed' && t.attachmentId && t.attachmentFilename) {
+        resolve({ attachmentId: t.attachmentId, filename: t.attachmentFilename });
+        return true;
+      }
+      if (t.state === 'failed' || t.state === 'aborted') {
+        reject(new Error(t.error?.message ?? 'Upload failed'));
+        return true;
+      }
+      return false;
+    };
+
+    if (check()) return;
+
+    const unsub = useTransferStore.subscribe(() => {
+      if (check()) unsub();
+    });
+  });
+}