Add menu editor

_Originally: https://github.com/virge-io/pricelist-frontend/issues/476_

# What should the editor do:

- [ ] The client should be able to change the background image of a show
- [ ] The client should be able to add icons like an 18+ warning
- [ ] The client should be able to customize their show however they want (product order, layout, etc.)

# How we will do it:

- [ ] This still needs to be investigated

No files, tests, or entry points are named. Start by investigating where show menus are edited, then clarify the scope and acceptance criteria for background images, icons, product ordering, and layout customization; done requires an agreed implementation plan for each capability.

索引モデルが issue の本文から書いたものです。