Import preview and recovery

## Goal

An import is a one-shot transformation that touches a lot of content. People want to see what's about to happen before they commit, and to back out if something looks off. Add a preview before the import runs and a rollback for the entire batch.

## Scope

* Before running, show a preview with the counts: how many pages, collections, rows, and unsupported items the export contains.
* During the import, show progress and surface failures clearly.
* After the import, let the user retry failed items or roll back the whole batch as a single undo.

## Out of scope

* Selective rollback of individual items; rollback is per batch.
* Export (different concern).

## Acceptance criteria

* Uploading an export shows a preview before the user commits.
* The import shows progress while running and a clear summary of failures at the end.
* A user can retry failed items or roll back the entire batch from the summary.

No files, tests, or entry points are named; start by locating the existing import flow and its upload, commit, and summary UI. Compare that flow with the acceptance criteria: preview counts, progress, failure summary with retry, and whole-batch rollback. Done means all three behaviors work without adding selective rollback or export changes.

Written by the indexing model from the issue text.