Rivermaze, I can relate just a bit with your dismay as I've watched my own sister's 20 years of severe depression degenerate into dementia. She's tried every medical treatment possible but wouldn't consider taking any advice or help from me or making any lifestyle changes that might have gotten to the root of the problem. I think we all see people struggle in life, some with huge challenges. But why do certain people overcome their issues while others don't? It doesn't seem to be the degree of life's difficulties since there''s abundant movies, books, and videos of people overcoming major adversities. There seems to be an underlying self identity or character trait that allows some to succeed while others fail. To that point, I see a difference between managing or treating depression versus resolving or preventing it. As a Christian, I tend to favor the latter as a desirable goal.
The good news is that you have a better chance of helping or influencing your partner than I have with my sister, assuming you're physically together on a daily or frequent basis. What to do then? Well, I won't discount the suggestions that others have made here, but they all require the other person to take action and do things, which doesn't work if the other person is unmotivated to do so. Sure, a depressed person will feel better for awhile if you get them out of the house for some fun or nature, but that won't solve the underlying problem. Disinterest in life will continue until something more important, a deficiency of some sort, is addressed.
I understand that some forms of depression are caused by chemical imbalances in the body that can be helped with modern medicine and/or diet. I assume though that your partner has gone through this assessment and not found this to be the resolvable problem.
I understand that some forms of depression are caused by being trapped in a terrible, negative environment. This was the case for my sister, but she was unwilling to leave or change that environment, so no cure was possible. I assume your partner is not in such a situation or you would have mentioned it.
That leaves the tough cases of depression concerning the mind and soul. I think these matters are determined by our sense of identity, self worth, and purpose in life. To these points, a true Christian has a solid foundation to live by. Knowing and loving God as we were created to do gives one a strong sense of value, purpose, peace, and joy in life. In short, when God's Spirit fills our mind and soul, there's no room for depression.
So how could you help your partner change their mind and soul? I'd suggest leading by example. Dedicate more time and attention to knowing and loving the Lord each day, and your passion, peace, and joy will grow and become apparent to others around you. Let your life show something that your partner wants. Such influence can cause real life changing effects.