fix(d3fend): use d3fend_id as expand key instead of def.id

def.id was undefined for D3FEND items, causing expandedId===undefined
to match ALL items simultaneously and opening every card at once.
d3fend_id (e.g. 'D3-DSDP') is always present and unique per defense.

frontend/src/pages/TechniqueDetailPage.tsx

@@ -880,14 +880,14 @@ function D3FENDSection({ defenses }: { defenses: Array<{
           )}
           <div className="grid gap-2 sm:grid-cols-2">
             {defs.map((def) => {
-              const isExpanded = expandedId === def.id;
+              const isExpanded = expandedId === def.d3fend_id;
               return (
                 <div
-                  key={def.id}
+                  key={def.d3fend_id}
                   className={`rounded-lg border p-3 transition-all cursor-pointer ${
                     isExpanded ? "ring-1 ring-gray-600" : ""
                   } ${tacticColors[tactic] || "border-gray-700 bg-gray-800/30 text-gray-300"}`}
-                  onClick={() => setExpandedId(isExpanded ? null : def.id)}
+                  onClick={() => setExpandedId(isExpanded ? null : def.d3fend_id)}
                 >
                   <div className="flex items-start justify-between">
                     <div className="min-w-0 flex-1">