fix(gui): pin slint to 1.15 to avoid muda duplicate-class link error

slint 1.16 changed its muda dep from 0.17 to 0.18, while tray-icon 0.22
stayed on 0.17. Both versions register the Objective-C class
`__CLASS_MudaMenuDelegate`, causing the macOS `vykar-gui` release
binary to fail at link time:

  Linking globals named '__CLASS_MudaMenuDelegate': symbol multiply defined!
  error: failed to load bitcode of module "muda-…"

tray-icon 0.23/0.24 use muda 0.19 (still ahead of slint 1.16's 0.18),
and slint 1.16 is the latest published. Constrain slint and slint-build
to `~1.15` until either slint catches up to muda 0.19 or a slint
version using a matching muda is released. tray-icon stays at 0.22.2.

Author: m3nu