fix(board): restore container grid geometry (#6808)

apps/nextjs/src/components/board/sections/grid/section-grid.module.css

@@ -46,7 +46,6 @@
 
 .editorGrid {
   position: absolute;
-  inset: 0 auto auto 0;
 }
 
 .editorPortalHost {
@@ -63,6 +62,11 @@
   overflow: visible;
 }
 
+/* Containers provide their own inner item gap, so their card must keep the full grid aspect ratio. */
+.staticItem[data-type="section"] > .contentMount {
+  inset: 0;
+}
+
 .staticItem[data-type="item"] > .contentMount {
   inset: calc(10px * var(--board-canvas-inverse-scale, 1));
 }