Add unsigned desktop releases for macOS, Windows, and Ubuntu

[mrdjango]

Summary

• add production Electron packaging with electron-builder 26.0.12
• build macOS arm64/x64, Windows x64, and Ubuntu x64 artifacts on native GitHub-hosted runners
• support manual artifact builds and tag-triggered draft GitHub releases
• publish SHA256SUMS and explicit unsigned-build installation warnings
• keep signing, notarization, and Windows executable signing disabled for this first release phase
• add structural contract tests for packaging targets and the release workflow

Why

Tcode needs repeatable downloadable desktop builds for macOS, Windows, and Ubuntu. Keeping the packaging configuration in this private standalone repository makes releases independently maintainable while certificates and signing infrastructure are deferred.

Validation

• npm run test:desktop-release-config: 4 passing
• 83 Theia package test targets passed sequentially
• Electron application tests: 2 passing
• Browser API tests: 838 passing
• exact macOS release command produced a 211 MB DMG and 204 MB ZIP
• git diff --check passed
• graphify knowledge graph refreshed

Notes

The generated applications are intentionally unsigned. macOS Gatekeeper and Windows SmartScreen may warn users on first launch. The release workflow includes SHA-256 checksums so downloads can be verified.