Who has the final say in whether a law or treaty violates the constitution? The U.S. House of Represenatives, the U.S. Senate, The President of the United States of America, or the U.S. Supreme court?

While the others may be involved in creating laws, I believe only the Supreme Court decides if a law is constitutional.

And also the lower courts.

Not all decisions go to the Supreme Court.

Sometimes the lower courts make the ruling and it does not get appealed to the Supreme Court (or it might get appealed, but the Supreme Court can refuse to hear the case).

The "final" say resides in the Supreme Court. However, that power is not specified in the Constitution. It was Chief Justice John Marshall, who interpreted the Constitution (specifically Article III) to invest the power of judicial review with the Judicial branch. He set the precedent in Marbury v Madison. From then on, it became the province of the Judicial branch to rule on the constitutionality of laws.

The President can appoint Supreme Court justices that are favorably disposed toward his views and in that way prevent his actions from being declared unconstitutional when it comes down to a Supreme Court decision. AS for not being able to declare war without the consent of Congress, that's no obstacle to going in and fighting an undeclared war. Which makes that particular law worthless since it doesn't stop the Commander In Chief from doing whatever he needs to do with his armed forces at a moment's notice and gives him the angle he needs to evade the Supreme Court unconstitutional declaration of his actions threat.