feat(realtime): live online/offline events and unified search

- add websocket events user_online/user_offline
- broadcast presence changes on first connect and final disconnect only
- apply live presence updates in web chat store and realtime hook
- move public discover into unified left search (users + groups/channels)
- remove separate Discover Chats dialog/menu entry

web/src/store/chatStore.ts

@@ -37,6 +37,7 @@ interface ChatState {
   setTypingUsers: (chatId: number, userIds: number[]) => void;
   setReplyToMessage: (chatId: number, message: Message | null) => void;
   updateChatPinnedMessage: (chatId: number, pinnedMessageId: number | null) => void;
+  applyPresenceEvent: (chatId: number, userId: number, isOnline: boolean, lastSeenAt?: string) => void;
 }
 
 export const useChatStore = create<ChatState>((set, get) => ({
@@ -244,5 +245,32 @@ export const useChatStore = create<ChatState>((set, get) => ({
   updateChatPinnedMessage: (chatId, pinnedMessageId) =>
     set((state) => ({
       chats: state.chats.map((chat) => (chat.id === chatId ? { ...chat, pinned_message_id: pinnedMessageId } : chat))
+    })),
+  applyPresenceEvent: (chatId, userId, isOnline, lastSeenAt) =>
+    set((state) => ({
+      chats: state.chats.map((chat) => {
+        if (chat.id !== chatId) {
+          return chat;
+        }
+        if (chat.type === "private" && chat.counterpart_user_id === userId) {
+          return {
+            ...chat,
+            counterpart_is_online: isOnline,
+            counterpart_last_seen_at: isOnline ? chat.counterpart_last_seen_at : (lastSeenAt ?? new Date().toISOString())
+          };
+        }
+        if (chat.type === "group") {
+          const currentOnline = chat.online_count ?? 0;
+          const membersCount = chat.members_count ?? currentOnline;
+          const nextOnline = isOnline
+            ? Math.min(membersCount, currentOnline + 1)
+            : Math.max(0, currentOnline - 1);
+          return {
+            ...chat,
+            online_count: nextOnline
+          };
+        }
+        return chat;
+      })
     }))
 }));