Release openmozi v1.4.0 with macOS downloads

[spytensor]

What changed

• adds browser-native PowerPoint previews
• publishes lowercase openmozi DMG, ZIP, manifest, and checksums to GitHub Releases
• makes provider compatibility checks manual-only and bounds the Unit job
• updates the public version and changelog to v1.4.0

User impact

The Release page will contain downloadable macOS app files. Unsigned builds are clearly marked as prereleases, while the installed application remains MOZI.app.

Validation

• clean frozen-lockfile install
• public privacy and commit-message gate
• gitleaks directory and commit scans
• 72 targeted tests passed
• typecheck
• server build
• UI production build
• desktop build
• workflow YAML parse and diff check