fix(d3fend): add items-start to grid so cards don't stretch to row height

CSS grid was stretching both cells in a row to the same height as the
tallest card, making the unexpanded card appear 'open' with blank space.
items-start makes each card only as tall as its own content.

frontend/src/pages/TechniqueDetailPage.tsx

@@ -878,7 +878,7 @@ function D3FENDSection({ defenses }: { defenses: Array<{
           {tacticDescriptions[tactic] && (
             <p className="mb-2 text-xs text-gray-500">{tacticDescriptions[tactic]}</p>
           )}
-          <div className="grid gap-2 sm:grid-cols-2">
+          <div className="grid gap-2 sm:grid-cols-2 items-start">
             {defs.map((def) => {
               const isExpanded = expandedId === def.d3fend_id;
               return (