fix(nav): no "bare content" on fast/repeated back-forward
The popstate handler fired an htmx.ajax immediately on every back/forward →
on fast repeated navigation requests overlapped and a slow old response could
overwrite a newer page ("bare content"). Now: doNav() aborts any pending
request on #pcms-main first, and popstate is debounced (90ms) so fast
repeated back-forward only fetches the LAST destination. Click navigation
uses the same abort-first doNav().
Co-Authored-By: Claude <noreply@…>