BUY listings can come back from the API with null `total_price`, `qm`,
`qmprice`, `rooms`, and `last_seen` even though the TypeScript types
declare them non-nullable. The cards called `.toLocaleString()` and
`.split('T')` on those values, throwing TypeError and tripping the
ErrorBoundary — which is the "BUY part of the page crashes" symptom.
Coerce numeric fields via a `safeNum` helper (0 fallback), gate the
"Nd ago" line on a finite last_seen, and apply the same guards in
PropertyCard, PropertyCardCompact, SwipeCard, and the price-history
section of ListingDetail. Added regression tests asserting both card
variants render with all-null backend fields without throwing.
