Is it wrong to plan for the future, such as saying "I will do this tomorrow, or next month, or next year," while believing that death could come at any time?

There is no impediment to a person planning for the future and estimating their needs and goals, provided they say "insha'Allah" (God willing); based on the Almighty's saying: (And never say of anything, "Indeed, I will do that tomorrow," except [by adding], "If Allah wills.") Hope motivates a person to work, and a believer strives for what benefits them in their religion and worldly life, while taking the necessary means, relying on Allah, and seeking His help; based on the Prophet's (peace be upon him) saying: "Be keen on what benefits you and seek help from Allah." As for the disbeliever and the heedless, they rely on means without relying on Allah. Hope is essential for life, but a believer should be short in hope, not settling for this world, and keeping the Hereafter in sight, thus striving in righteous deeds.