Integrate idle todo prompts with current settings UI

[jasonsuhari]

Problem

The existing feat/settings-todo-prompts branch needs to be rebased/integrated onto the current restyled settings work and recent pane/runtime/worktree/footer changes.

Requested behavior

Bring idle todo/follow-up prompt settings into the current main UI without regressing restyled settings, pane rename, sleep, usage labels, runtime resize, worktree management, or conversation footers.

Acceptance checks

• Integration branch starts from latest origin/main.
• Idle todo/follow-up prompt behavior is available in the current settings UI.
• Relevant conflicts are resolved against current main behavior.
• npm test passes.
• Branch is pushed and a PR to main is opened.