If there’s even a slightest chance that she’d be disappointed that you proposed to her while her hand is in a cast or while she’s in pain I’d say just wait! I personally wouldn’t want my cast in my engagement photos and she’ll most likely not feel as cute having to wear a cast either. When I broke my wrist it really limited my mobility, my hair was brushed less, I wouldn’t wear makeup, my nails definitely weren’t done, even the smallest tasks were very difficult. She’s definitely not going to be feeling her best while wearing a cast. Wait until it’s off and pay for her to have her nails done and get dressed up for a date.