the editor sections fall to one column when it is narrow

A container query on the scroller, once, so a new section is responsive by being
in the column rather than by remembering to say so. Below 320px the two-column
sections stack instead of truncating every select to "Martia…".

Colors stays flex-wrap: three swatches in a two-column grid is 2 and 1.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

web/sections/SpacingTab.tsx

@@ -94,7 +94,7 @@ export const SpacingTab = ({
       {BANDS.map((band) => (
         <div key={band.key} className="flex flex-col gap-2">
           <h4 className="text-xs font-medium">{band.label}</h4>
-          <div className="grid grid-cols-2 gap-3">
+          <div className="grid grid-cols-1 gap-3 @[20rem]:grid-cols-2">
             {band.fields.map((field) => {
               const current = (spacing as Record<string, Record<string, Steps> | undefined>)[band.key]?.[field.key];
               const own = current?.[key];