Schedule follow starts and prevent ImGui ID collisions

[wnj00524]

Summary

• Let users choose the simulation day and time when a follow begins, while keeping immediate start as the default.
• Keep scheduled assignments pending until their start time; travel, observations, and follow duration start then.
• Give scheduler controls distinct ImGui labels and scope report rows by index to prevent duplicate IDs.
• Document the scheduling and GUI ID behavior.

Closes #167

Validation

• git diff --check passed.
• Tests were not run locally.