Cross-fade Active Fires markers and heatmap; fix the spot size

The far-zoom heatmap now uses the ½× grid (4096 × 2048, ≈10 km per
pixel, one-pixel blur), so a lone fire's spot matches its marker at the
3,000 km hand-off. The spot size slider and its share option are gone.

Markers and heatmap are both kept built at every zoom and cross-fade by
camera height between 2,400 and 3,600 km on every frame, so a fast zoom
never shows a beat with neither. The heatmap fades by imagery alpha:
hiding a Cesium imagery layer unmaps it from the globe tiles, and showing
it again re-mapped them late, which was the gap at the band edge. Under
the markers the heatmap is built 150 ms after the data or filter changes.

src/i18n/locales/es.json

@@ -2993,9 +2993,6 @@
   "≥ {value} MW": "≥ {value} MW",
   "Confidence": "Confianza",
   "Minimum fire radiative power, megawatts": "Potencia radiativa mínima del fuego, en megavatios",
-  "Heatmap spot size": "Tamaño de las manchas del mapa de calor",
-  "Far-zoom heatmap spot size": "Tamaño de las manchas del mapa de calor en la vista lejana",
-  "{value}×": "{value}×",
   "fire intensity filter|All": "Todo",
   "Detection confidence": "Confianza de la detección",
   "{shown} of {total} shown": "{shown} de {total} mostrados",