fix: sqlite3_open wrong path in topo_group.c (db_path vs db_path/chats.db), keep separate connections for perf

- topo_group.c: open {db_path}/chats.db instead of {db_path} — was opening a directory
- chat_core.c: remove inst->topo_sqlite_db = g_cc.db — keep topo_sqlite_db as separate
  uTun-only connection without FULLMUTEX, avoiding contention with GUI reads

tools/chatgui/transport/chat_core.c

@@ -105,7 +105,6 @@ int chat_core_init(struct UTUN_INSTANCE* inst, const char* db_path) {
     g_cc.my_node_id = inst->node_id;
 
     topo_node_sqlite_init(g_cc.db);
-    inst->topo_sqlite_db = g_cc.db;
 
     /* записать себя в nodes + local_identity с реальным node_id и именем */
     {