You have no idea if this is what OP is planning. Our wedding certainly wasn't like this
Nor was ours. But as you say, we have no idea what sort of wedding OP is planning, only that she’s started planning it very early and friend is reacting negatively to talk of it.
Friend could be jealous but she may just be bored. IME friends planning weddings tend to get very fixated on talking about all the tiny details. From the menu to the rings, outfits, hair, make up, favours, invitations, arrival method, vows, seating plans, flower arrangements, hen do, decor etc. Costs for the hen do, hotels, wedding gifts, guest outfits etc also have a tendency to spiral out of control. Could friend be secretly annoyed at costs eg are you asking her to come on a hen do?
It’s possible OP is planning a very basic, pared down wedding but the fact she’s talking about it to her friend already (and friend is less than enthusiastic), suggests friend finds wedding talk very tedious.
Or she could be envious and feel OP is rubbing it in that’s she’s single.
OP I suggest you share your excitement with other women planning weddings, rather than expecting your friend to share your enthusiasm. Maybe she’s in a place where she simply can’t get enthusiasm over your wedding?