Add visitor read receipts

public/admin.js

@@ -435,10 +435,19 @@ function renderActive() {
       ${message.body ? `<p>${escapeHtml(message.body)}</p>` : ""}
       ${renderAttachments(message.attachments)}
     </article>
-  `).join("");
+  `).join("") + adminReadReceipt(conversation, messages);
   $("#messages").scrollTop = $("#messages").scrollHeight;
 }
 
+function adminReadReceipt(conversation, messages) {
+  const lastAdminMessage = [...messages].reverse().find((message) => message.senderType === "admin");
+  if (!lastAdminMessage) return "";
+  const seenAt = conversation.visitorSeenAt;
+  const isSeen = Boolean(seenAt && String(seenAt) >= String(lastAdminMessage.createdAt));
+  const text = isSeen ? `Precteno zakaznikem ${formatTime(seenAt)}` : "Zatim neprecteno";
+  return `<div class="read-receipt ${isSeen ? "seen" : "unseen"}">${escapeHtml(text)}</div>`;
+}
+
 function filteredConversations() {
   if (state.statusFilter === "all") return state.conversations;
   if (state.statusFilter === "active") return state.conversations.filter((item) => item.status === "new" || item.status === "open");
@@ -641,6 +650,14 @@ function formatVisitedAt(value) {
   return date.toLocaleTimeString("cs-CZ", { hour: "2-digit", minute: "2-digit" });
 }
 
+function formatTime(value) {
+  const text = String(value || "");
+  const normalized = /^\d{4}-\d{2}-\d{2} \d{2}:\d{2}:\d{2}$/.test(text) ? `${text.replace(" ", "T")}Z` : text;
+  const date = new Date(normalized);
+  if (Number.isNaN(date.getTime())) return "";
+  return date.toLocaleTimeString("cs-CZ", { hour: "2-digit", minute: "2-digit" });
+}
+
 function deviceLabel(device = {}, language, timezone) {
   return [device.platform, device.viewport, language, timezone].filter(Boolean).join(" | ");
 }