etcp: instance-level conn status callbacks (NEW/UP/DOWN/DELETE), fix member online in chatgui

- Add etcp_add_conn_status_cbk/remove/set — unified callback for all connections
- 4 statuses: NEW, UP, DOWN, DELETE — one subscription covers all connections
- Fire points in etcp.c: create→NEW, on_up→UP, on_down→DOWN, close→DELETE
- chat_sync.c: replace cs_on_new_conn+per-conn subs with cs_on_conn_status
- db_sync.c: same migration to instance-level callback
- cs_is_peer_online: use conn->links_up instead of link->link_status (was 0 during INIT handshake)

src/utun_instance.h

@@ -102,6 +102,7 @@ struct UTUN_INSTANCE {
     
     // Callback chain for new ETCP connections
     struct etcp_cbk_entry* new_conn_cbks;
+    struct etcp_status_cbk_entry* conn_status_cbks;  // instance-level: NEW/UP/DOWN/DELETE
     void* test_user_ptr;             // Generic user pointer (used by tests)
 
     struct memory_pool* data_pool;// для входных-выходных данных пакета