feat: multi-file upload queue for Add Document (#789)

Adds multi-select and multi-file drag-and-drop on the Add Document Upload files tab, a per-file queue with status, retry for failed rows, and batched uploads to the existing `POST /v3/documents/file` endpoint with a client-side concurrency limit of 3.

Optional title/description apply only when a single file is queued. .md / .mdx (and text/markdown) are accepted.

Upload progress is shown as a bottom strip with a slow width animation

Author: MaheshtheDev