Sign and notarize the macOS Tauri viewer build

[omscse]

The macOS viewer build is currently unsigned. macOS Gatekeeper will block it for users who download it outside the App Store.

Requirements:

• Apple Developer account ($99/year)
• Code signing certificate (Developer ID Application)
• Notarization via xcrun notarytool

Steps:

1. Enroll in Apple Developer Program
2. Generate Developer ID certificate
3. Configure Tauri tauri.conf.json signing fields
4. Add notarization step to build/release workflow
5. Verify Gatekeeper accepts the signed build: spctl --assess --type exec

Needed before Homebrew cask distribution.