Fix: sidebar resize handle hit area

[SylTi]

Summary

• replace the outer sidebar GtkPaned with an explicit 3px resize handle so the resize hit area matches the
  visible handle
• keep sidebar toggle and session restore behavior by tracking width on the sidebar shell and hiding the
  shell and handle together
• extract sidebar split construction and resize hookup into helpers to keep build_window smaller without
  changing behavior

Testing

• cargo build --release -p limux-host-linux
• manual check: the first terminal character is selectable
• manual check: the left sidebar still resizes from the visible handle