add firebase and sign up

src/lib/layout/selection.svelte.js

@@ -1,18 +1,46 @@
+import { db } from '../firebase.js';
+import { doc, onSnapshot, updateDoc, arrayUnion, arrayRemove } from 'firebase/firestore';
+
 let selected = $state(new Set());
 let totalCountries = $state(0);
+let _uid = null;
+let _unsubscribe = null;
+
+export function initSelectionListener(uid) {
+  if (_unsubscribe) _unsubscribe();
+  _uid = uid;
+  const userRef = doc(db, 'users', uid);
+  _unsubscribe = onSnapshot(userRef, (snap) => {
+    if (snap.exists()) {
+      const codes = snap.data().visitedCountries || [];
+      selected = new Set(codes);
+    }
+  });
+}
 
 export function toggle(id) {
+  if (!_uid) return;
+  const was = selected.has(id);
   const next = new Set(selected);
-  if (next.has(id)) {
+  if (was) {
     next.delete(id);
   } else {
     next.add(id);
   }
   selected = next;
+  const userRef = doc(db, 'users', _uid);
+  if (was) {
+    updateDoc(userRef, { visitedCountries: arrayRemove(id) });
+  } else {
+    updateDoc(userRef, { visitedCountries: arrayUnion(id) });
+  }
 }
 
 export function clearAll() {
+  if (!_uid) return;
   selected = new Set();
+  const userRef = doc(db, 'users', _uid);
+  updateDoc(userRef, { visitedCountries: [] });
 }
 
 export function getSelected() {