fix sidebar active thread scrolling

[tarik02]

What Changed

• scroll to the active thread instantly when the sidebar appears
• smooth-scroll only when thread navigation originates outside the sidebar
• remove draft-project scrolling and the previous pending-target state machine

Why

Direct sidebar navigation already starts from a visible row, so scrolling it again moves the list for no reason. External navigation, including command palette selection and deletion fallback, still needs to reveal the new active thread.

UI Changes

The sidebar no longer moves after selecting a thread inside it. Opening the sidebar reveals the active thread without animation; external thread changes reveal it with smooth scrolling.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes
• I included a video for animation/interaction changes