Closes#18. Navigating away from a PO create/edit screen with unsaved
changes could silently lose in-progress work. The forms now track a dirty
flag and guard both navigation paths:
- Hard navigations (refresh / tab close / external link) → the browser's
native "Leave site?" prompt via beforeunload.
- In-app navigations (sidebar / header / any internal link) → a capture-phase
click interceptor opens a modal offering Save as draft / Discard changes /
Stay on page. Save as draft runs the form's existing draft save (which
redirects to the PO); Discard continues to the intended destination.
The guard (components/po/unsaved-changes-guard.tsx) is reusable and wired into
both new-po-form and edit-po-form. dirty is cleared before a successful submit
so saving never trips the prompt. SPA back-button (popstate) is left to
beforeunload only; the manager inline-edit panel is out of scope (saves in
place, no draft concept).
Tests: 7 new unit cases for the guard (intercept-when-dirty, no-op-when-clean,
external links pass through, Stay/Discard/Save actions, beforeunload arming).
Unit suite 296 green; tsc clean.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>