Persist navigation expansion and pane layout

[emsi]

Summary

• persist notebook and section-group expansion by stable, source-scoped identity
• restore the collapsed state of the Notebooks and Pages panes
• preserve the last page when a section moves or its legacy position-derived ID changes
• tolerate legacy, malformed, and unsupported optional UI state without losing the workspace

Motivation

OneNote Viewer already restored open sources and the last page, but every restart reset the navigation tree and pane layout. The prior section and group IDs were derived from tree positions, so they were also unsuitable for durable UI state: renaming, moving, or reordering an entry could redirect saved state.

This change adopts native OneStore file identities exposed by onenote.rs, scopes them to each source, and stores UI state in an optional versioned workspace record. Missing groups and closed or failed sources are pruned. Delayed source loading applies state only to the matching source and cannot replace a newer active selection.

The parser API is proposed upstream in msiemens/onenote.rs#45. Until that is merged together with the viewer's existing image/icon dependency, the viewer pins a signed integration revision from the project fork.

User impact

After restarting, the viewer restores expanded notebooks and section groups as well as collapsed navigation panes. Existing workspace files continue to load with the established defaults.

Closes #38.

Verification

• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt --all -- --check
• git diff --check