Add desktop updater and release packaging flow

[ksamaschke]

Summary

• add packaged desktop updater coordination and Electron bridge methods
• add electron-builder release packaging config and draft GitHub release workflow
• update dashboard release UX and desktop release verification tests/docs

Verification

• npm test
• npm run build:desktop:release
• npm run build:dashboard:web && node --test dist/tests/installer/api-realtime.test.js dist/tests/installer/control-plane-client.test.js dist/tests/installer/desktop-preview-startup.test.js

Notes

• local desktop packaging produced a macOS arm64 DMG and updater metadata
• local signing/notarization remained environment-limited because valid production credentials were not configured on this machine

[ksamaschke]

ICA-REVIEW-RECEIPT
Reviewer-Stage: 3 (temp checkout)
Reviewer-Agent: codex reviewer (subagent)
Head-SHA: c1f2436
Findings: 0
NO FINDINGS
Result: PASS
Evidence: Clean temp-checkout verification via npm test on the PR head SHA.