chore(release): prepare OffPDF v0.4.0

[McanKul]

Summary

• sync the release branch with main
• bump all release metadata to 0.4.0
• add the v0.4.0 changelog and release notes
• document the completed direct-editing foundation and its deliberate product boundary
• retain the Windows HEIC HEIC-to-PDF import crashes the application #14 known issue

Validation

• npm run check:versions
• npm run build
• npm test (349 passed)
• cargo test --manifest-path src-tauri/Cargo.toml --lib (325 passed)