Uploader: 'spread from' line shows the batch-wide start after spreading two folders

In the capture-times panel, spread folder A from 08:00 and folder B from 14:00. Every card in folder B reads "spread from 08:00", because `spreadStartOf` in `apps/sparcd-uploader/src/lib/useCaptureEstimates.ts` takes the earliest spread time in the whole batch. Cosmetic, the times themselves are right.

Fix: compute the start within the file's own folder, or persist the start alongside `manualSource` when the spread is applied.

Start in apps/sparcd-uploader/src/lib/useCaptureEstimates.ts and reproduce the capture-times panel case by spreading folder A from 08:00 and folder B from 14:00. Inspect spreadStartOf and verify that each card’s “spread from” label uses its own folder’s start while the displayed times remain correct.

Written by the indexing model from the issue text.