Render stored image fallbacks and attachment icons

[emsi]

Summary

• expose OneNote browser-compatible image representations and embedded-file icons as bounded lazy resources
• prefer a decodable primary image, then use the stored fallback without invoking external converters
• render stored attachment icons with a stable generic fallback and reject stale asynchronous texture results
• pin the signed parser-fork revision and refresh Flatpak sources

Verification

• cargo test --workspace --all-targets --locked
• cargo clippy --workspace --all-targets --locked -- -D warnings
• CI-equivalent isolated GTK/Xvfb tests
• ./scripts/check-licenses.sh
• release workflow 31257913553: native, Flatpak, and AppImage jobs passed

Closes #28