feat: open OneNote backup folders as notebooks

[emsi]

Summary

• add a reusable, bounded loader for manifest-free OneNote backup folders
• reconstruct nested directories as section groups and select the latest dated section snapshot by default
• expose manual refresh and an explicit option to show all backup copies
• preserve stable source identities, workspace state, transactional indexing, progress, and cancellation
• prevent foreground-operation completion from re-borrowing active UI state and aborting the application

User impact

OneNote desktop backup directories can now be opened as one searchable notebook instead of many unrelated one-section notebooks. Source files remain read-only and section order is clearly treated as reconstructed when the original .onetoc2 is absent.

Reliability

Traversal is bounded and symlink-safe. Loading detects source changes, keeps the last published generation on failure, and stages notebook publication until indexing succeeds. Foreground operation cleanup releases its state borrow before refreshing GTK actions.

Validation

• cargo test --workspace --all-targets --locked --offline
• cargo clippy --workspace --all-targets --all-features --locked --offline -- -D warnings
• GTK icon verification and startup smoke test under Xvfb
• GitHub release workflow 32056843291: native, Flatpak, and AppImage jobs passed
• manual Flatpak verification against a representative backup folder

Closes #39