chore: implement vectors

also seems to address several of these:
https://github.com/javalsai/lidm/issues/3#issuecomment-2423933131

not sure about the issue itself, might be permission stuff

include/ui.h

@@ -2,9 +2,10 @@
 #define _UIH_
 
 #include <config.h>
+#include <util.h>
 
 void setup(struct config);
-int load(struct users_list *, struct sessions_list *);
+int load(struct Vector * users, struct Vector * sessions);
 void print_err(const char *);
 void print_errno(const char *);