Both the PO create and edit screens now guard against accidental loss of
in-progress data. While a form has unsaved changes:
- in-app link clicks and the edit screen's Cancel button are intercepted,
showing a dialog offering Save as Draft / Discard changes / Stay on page
- hard unloads (reload, tab close, external navigation) trigger the browser's
native warning via beforeunload
"Save as Draft" reuses the existing draft-save action; validation errors are
surfaced in the dialog so the user can recover or discard. Dirty state is
tracked from any field edit, line-item change, or attachment change.
Adds useUnsavedChanges hook + UnsavedChangesDialog component and unit tests.
Fixes#18
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
