<<My dog’s front "knee" is slightly swollen and hard>>
The description is confusing, what you are describing, if on the front leg, is the carpal region or wrist of the dog. Unilateral lameness of a chronic nature (over 7 days) is something, as ZooCVT has stated, needs xrays. The stiffness in the morning and after play is supportive of degenerative changes in the carpus, but as you probably know, not the most common areas to have problems in a larger dog. The firm swelling of the joint can signify the connective tissue changes that are occuring to stabilize the joint as well as the intrajoint swellings.
For the night you can give a regular strength Aspirin with food and preferably with a tsp of Maalox 20 minutes prior to the Aspirin. This will help with the swelling and discomfort, but I recommend in support of ZooCVTs suggestion, that a vet exam and xray is needed.