You can put your source code in any file. ui.R and server.R has special meaning for shiny apps (they are the ones being executed by runApp()), but they can in turn source code from any file. I guess you somewhere in server.R has a source("dados.R") statement?
With the risk of beating a dead horse, are you absolutely sure that dados.R can see db.R? I know they are in the same folder, but are you sure that when dados.R is executed that getwd() returns "C:/Users/Maia/Desktop/Relatorios R QT/Escoamento_Capabilidade" ? One way to test this is to change your source statement in dados.R to include the full path, i.e.
source("C:/Users/Maia/Desktop/Relatorios R QT/Escoamento_Capabilidade/db.R")
(please check the spelling in the statement above, I have already edited it twice realizing typos :-))
If this works, your working directory may not be what you think it should be. If it doesn't work I think I am out of ideas...