Replace the single global max travel time filter with per-POI filters.
Each POI gets its own travel mode selector and max minutes input in the
filter panel. Listings must satisfy ALL active filters (AND logic).
Fix Mapbox "Input is not a number" error by ensuring color stops are
always strictly monotonic (guard min === max) and always set (even when
no valid metric values exist). Also filter Infinity values from the
color scale computation. Widen the filter panel from w-64 to w-80.
