add side stats panel

src/lib/layout/selection.svelte.js

@@ -0,0 +1,28 @@
+let selected = $state(new Set());
+let totalCountries = $state(0);
+
+export function toggle(id) {
+  const next = new Set(selected);
+  if (next.has(id)) {
+    next.delete(id);
+  } else {
+    next.add(id);
+  }
+  selected = next;
+}
+
+export function clearAll() {
+  selected = new Set();
+}
+
+export function getSelected() {
+  return selected;
+}
+
+export function setTotalCount(n) {
+  totalCountries = n;
+}
+
+export function getTotalCount() {
+  return totalCountries;
+}