fix(web): do not name a restore time from the browser clock

The confirm text now says "Za 2 godz. wrócimy do X"; the exact time
comes from the gateway's restoreAt in the result after the write.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

backend/src/main/resources/web/settings.js

@@ -134,8 +134,8 @@ function saveBox(data) {
 function saveSummary(s) {
   const value = degrees(state.draft.value);
   if (!state.hours) return `Ustawię ${value} do odwołania. Poprzednia wartość (${degrees(s.value)}) nie wróci sama.`;
-  const back = when(new Date(Date.now() + state.hours * 3600000).toISOString());
-  return `Ustawię ${value}. O ${back} wrócimy do ${degrees(s.value)}.`;
+  // The exact time is the gateway's to name, from its clock, in the result after the write.
+  return `Ustawię ${value}. Za ${duration(state.hours)} wrócimy do ${degrees(s.value)}.`;
 }
 
 // A refresh can lose the reading while a change is half-made; drop what was built on it.