I work remotely and often sit in a chair for hours. sometimes my feet/ankles will be swollen at the end of the day. can this mean heart failure or just that i need to be sure to exercise periodically instead of sitting? i am otherwise healthy?

That finding isn't uncommon, usually noted more in certain people with mild venous insufficiency or women on oral contraceptive. Yes, you can alleviate the edema with either flexing your feet up and down 5-10 times, a couple times/hour when working or walking around or raising your feet up during break time. You are otherwise healthy so you shouldn't be occupied with that unfounded concern.