Resumable uploads for large files

Large .mbox / .pst uploads (often multi-GB) can be interrupted partway — a dropped/slow connection, a closed tab, or a session timeout — and currently the user has to start the whole upload again from scratch. For big mailboxes this is painful and a common drop-off point.

Goal: chunked / resumable uploads so an interrupted transfer continues from where it stopped instead of restarting.

Related to the mid-upload session keep-alive work (which mitigates the logout case for slow connections, but does not make the upload itself resumable). Tracking this as the durable follow-up.