Add safe on-demand attachment actions

[emsi]

Summary

• add a reusable bounded, cancellable streaming API for lazy OneNote resources
• add explicit Open and Save As... attachment actions with progress and cancellation
• use GIO/portal file handling, private cache storage, safe filenames, and guarded destination publication
• add theme-aware pointer and keyboard interaction for attachment objects
• document the ownership boundaries, limits, and remaining accessibility scope

Safety and compatibility

• notebook sources remain read-only and attachments are never extracted eagerly
• attachment payloads are capped at 2 GiB and copied with a fixed 64 KiB buffer
• unsafe display names cannot become paths, and interrupted writes do not replace existing files
• Open uses a private XDG cache and the desktop launcher path
• no broad Flatpak filesystem permission is added

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --locked --offline -- -D warnings
• cargo test --workspace --all-targets --locked --offline
• GTK_A11Y=none GSK_RENDERER=cairo xvfb-run -a cargo test -p onenote-render-gtk attachment_ --locked --offline
• ./scripts/check-licenses.sh
• ./scripts/update-flatpak-sources.sh
• git diff --check

Closes #16