Capture user-facing dialogs in installed-app screenshots

[kostyafarber]

Problem

The installed-app screenshot workflow captures only a generic native file chooser, so reviewers cannot inspect the user-facing document, update, preview, settings-failure, and React error states changed by copy or UI work.

An initial expansion attempt reached the React error scenario but timed out on macOS, Windows, and Linux because the packaged screenshot application did not expose the E2E-only render-failure route. The screenshot harness needs a reliable boundary for installed builds rather than assuming the ordinary E2E build flags survive packaging.

Evidence: https://github.com/shift-editor/shift/actions/runs/33982007781

Expected outcome

Applying ci: installed app screenshots produces reviewable cross-platform screenshots of the actual dialog and failure implementations without changing Release or Nightly behavior.

Acceptance criteria

• The workflow captures file dialogs; document and update message boxes; both preview notices; collapsed and expanded application/document React errors; available, downloading, and ready update windows; and one representative settings save failure.
• macOS arm64, Windows x64, and Linux x64 capture jobs complete and publish their available scenarios in the sticky pull-request comment.
• Screenshot-only hooks or routes cannot be enabled in Release or Nightly packages.
• Captured copy comes from the shared message catalog and does not duplicate independently maintained user-facing text.
• Automated checks keep the scenario manifest, generated files, and report rows in sync.