add original matrix board to monitor

public/matrix.js

@@ -0,0 +1,23 @@
+"use strict"
+
+function setup() {
+  let highlighted
+  document.addEventListener("click", (event) => {
+    const card = event.target
+    if (!card.classList.contains("card") || card.classList.contains("cat")) {
+      return
+    }
+    if (card === highlighted) {
+      highlighted = null
+      card.classList.add("hidden")
+      return
+    }
+    if (highlighted) {
+      highlighted.classList.remove("highlight")
+    }
+    highlighted = card
+    card.classList.add("highlight")
+  })
+}
+
+setup()