Feature: Option to toggle curved UI / adjust curvature on small screens

Requested by Mariel Abbott in Ditto User Group.

After the curved UI overhaul, Mariel noted that on small mobile screens the curves make the UI feel cluttered. She suggested:

1. An option to toggle the curved UI on/off
2. Potentially making curvature a theme-level or per-user setting
3. Exploring more creative curve options (e.g. shallow sine wave)

Chad noted this is technically possible but needs a decision on whether it's an app-level or theme-level attribute. For now, the team is focused on finding a sane default.

Related context: The Saturn design philosophy motivates the curve (rings of Saturn), so a toggle would let users who prefer a cleaner layout opt out while keeping the default on-brand.