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
fix: space deletion + new space not reflecting in selector
Deleting a space wasn't switching back to Nova Spaces because we were reading stale cache data
New spaces created from the add-document modal weren't showing up in the dropdown
Added single-select mode to the space selector in the add-document flow since you're picking one space, not multiple.
