Persist navigation expansion and pane layout across restarts

[emsi]

Problem

Workspace state stores source paths and the last active page, but not expanded notebooks and section groups or collapsed navigation/page panes. Useful navigation context is lost after restart.

Acceptance criteria

• Expanded notebooks and section groups restore by stable source-scoped identity, never row index or label.
• Navigation and page-pane collapsed state restore.
• State belonging to closed sources is removed.
• Missing groups, schema upgrades, and corrupt optional state fall back safely.
• Delayed loading cannot overwrite newer user navigation.
• Tests cover renamed, removed, reordered, and duplicate-label groups.