[feat] expose snowflake in preferences

gui/api.md

gui/backend.go

gui/components/Preferences.qml

gui/gui.qrc

gui/handlers.cpp

gui/handlers.h

pkg/backend/api.go

pkg/backend/status.go

pkg/bitmask/bitmask.go

pkg/config/gui.go

pkg/snowflake/tor.go

pkg/vpn/main.go