fix(channel): enforce read-only for members and polish chat info

- block send/forward in channels for member role on backend
- expose my_role in chat payload for client-side permissions
- hide message composer for channel members and show read-only notice
- hide members management sections for private/saved chats in info panel
- return enriched chat detail via serialize_chat_for_user

web/src/pages/ChatsPage.tsx

@@ -17,6 +17,7 @@ export function ChatsPage() {
   const setActiveChatId = useChatStore((s) => s.setActiveChatId);
   const loadMessages = useChatStore((s) => s.loadMessages);
   const activeChat = chats.find((chat) => chat.id === activeChatId);
+  const isReadOnlyChannel = Boolean(activeChat && activeChat.type === "channel" && activeChat.my_role === "member");
   const [infoOpen, setInfoOpen] = useState(false);
 
   useRealtime();
@@ -61,8 +62,12 @@ export function ChatsPage() {
           <div className="min-h-0 flex-1">
             <MessageList />
           </div>
-          {activeChatId ? (
+          {activeChatId && !isReadOnlyChannel ? (
             <MessageComposer />
+          ) : activeChatId && isReadOnlyChannel ? (
+            <div className="border-t border-slate-700/50 bg-slate-900/40 p-4 text-center text-sm text-slate-300/80">
+              Только администраторы могут публиковать сообщения в этом канале
+            </div>
           ) : (
             <div className="border-t border-slate-700/50 bg-slate-900/40 p-4 text-center text-sm text-slate-300/80">
               Выберите чат, чтобы начать переписку