UX second pass: icon-only header, pane-toggle cue, schema columns, facet cleanup

src/app/components/DatasetsModal.module.css

@@ -235,12 +235,17 @@
   color: var(--text);
 }
 
+/* Two-up multi-column flow (min 220px per column, so a narrow pane falls back
+   to one): a wide schema reads down-then-over and a 25-field dataset no longer
+   buries the preview table below a full-screen scroll (NN/g #8;
+   docs/ux-second-pass.md resolution 2026-06-13). */
 .columns {
   list-style: none;
   margin: 0;
   padding: 0;
-  display: flex;
-  flex-direction: column;
+  columns: 220px 2;
+  column-gap: 0;
+  column-rule: var(--border-width) solid var(--border);
   border: var(--border-width) solid var(--border);
 }
 
@@ -250,10 +255,11 @@
   justify-content: space-between;
   gap: var(--space-3);
   padding: var(--space-2) var(--space-3);
-}
-
-.column + .column {
+  break-inside: avoid;
+  /* Hairline between rows; the negative margin tucks each column's first border
+     under the container border (a `+`-sibling border would double there). */
   border-top: var(--border-width) solid var(--border);
+  margin-top: calc(-1 * var(--border-width));
 }
 
 .columnName {