Add a heatmap spot size slider to the Active Fires row

A ½×–4× slider (log2 axis, 5 % steps, default 1×) under the intensity
filter sets how large far-zoom heat spots are. It scales the grid, not
the blur: 2× is a 1024 × 512 grid with the same one-pixel blur, so each
spot grows exactly and weak lone fires don't fade as they widen. A change
rebuilds only the heatmap. Share links keep it as w.s (percent, 50–400).
The layer row now takes a list of sliders.

src/share/viewCodec.js

@@ -252,7 +252,11 @@ export const SHARE_VIEW_FLAGS = freezeAll([
  * written explicitly.
  */
 export const SHARE_VIEW_LAYER_OPTION_ABSENT = Object.freeze({
-  'local-firms': Object.freeze({ minFrp: 0, confidence: 'all' }),
+  'local-firms': Object.freeze({
+    minFrp: 0,
+    confidence: 'all',
+    spotSize: 100,
+  }),
   traffic: Object.freeze({ roadMode: null }),
   'weather-lightning': Object.freeze({ opacity: 'strong' }),
   'weather-radar': Object.freeze({ opacity: 'strong' }),