App styles / alternate UI skins

[LizAinslie]

Summary

Optional alternate app styles (full UI skins), similar to Symfonium’s style picker.

Status: parked / not ready to implement yet — tracked so it stays on the roadmap.

Acceptance criteria (when unparked)

• Settings entry to choose style
• At least default + one alternate style
• Styles compose with dynamic color modes without fighting each other

Notes

Do not start implementation until explicit go-ahead. Prefer finishing dynamic color modes and M3 settings first.