Create chat modal UI & session login for WS auth

Backend: initialize a Django session on token obtain (login) so AuthMiddlewareStack can authenticate WebSocket connections; validate the token serializer and map TokenError -> InvalidToken; call django_logout on logout to destroy the session. Frontend: add a CreateChatModal component (DM/group creation) with user search, selection, validation, API mutation and cache invalidation; wire modal into ChatSidebar and add Czech translations for the new UI strings.

frontend/src/components/social/chat/ChatSidebar.tsx

@@ -7,10 +7,12 @@ import Avatar from "@/components/ui/Avatar";
 import Spinner from "@/components/ui/Spinner";
 import EmptyState from "@/components/ui/EmptyState";
 import IconButton from "@/components/ui/IconButton";
+import CreateChatModal from "./CreateChatModal";
 
 export default function ChatSidebar() {
   const { t } = useTranslation("social");
   const [query, setQuery] = useState("");
+  const [createOpen, setCreateOpen] = useState(false);
 
   const { data, isLoading } = useApiSocialChatsList(
     query ? { search: query } : undefined,
@@ -24,7 +26,11 @@ export default function ChatSidebar() {
         <h2 className="text-sm font-semibold text-brand-text">
           {t("chat.sidebar.title")}
         </h2>
-        <IconButton icon={<FiPlus size={16} />} label={t("chat.sidebar.new")} />
+        <IconButton
+          icon={<FiPlus size={16} />}
+          label={t("chat.sidebar.new")}
+          onClick={() => setCreateOpen(true)}
+        />
       </header>
 
       <div className="relative px-3 py-2">
@@ -86,6 +92,11 @@ export default function ChatSidebar() {
           ))}
         </ul>
       </div>
+
+      <CreateChatModal
+        open={createOpen}
+        onClose={() => setCreateOpen(false)}
+      />
     </aside>
   );
 }