Capturing 20 pages takes about ten seconds, and the capture switches the
app's coordinates to the screenshot's the whole time: a stroke drawn on
the canvas meanwhile would land in the wrong place. A modal dialog now
says which page is rendering and offers Cancel, which stops before the
next page. The open-page shot stays as it was, about a second with only
the spinner.

Saving into a folder now claims each name by creating the file
exclusively, so another export writing there at the same moment can't
replace it.

Tests: pages carry different content so a capture of the wrong page
fails, a cancel stops before the next page, and the panel test only
excuses the test font's overflow, not every error.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>