Distribute DPlex via Homebrew Cask

[Ron537]

Summary

Add a Homebrew Cask formula so macOS users can install DPlex with brew install --cask dplex. Cask handles the xattr quarantine flag automatically, removing the "is damaged" / right-click → Open friction for unsigned builds.

Acceptance criteria

• Create a new GitHub repo: Ron537/homebrew-dplex (small tap repo).
• Add Casks/dplex.rb with the cask definition pointing at the latest GitHub Release .dmg (use livecheck to track https://github.com/Ron537/DPlex/releases/latest).
• Compute and pin the SHA-256 of the latest .dmg (cask requires it).
• CI workflow in the tap repo that bumps the cask on every DPlex release (use action-homebrew-bump-formula or similar).
• Add a one-liner to the main repo README: brew install --cask Ron537/dplex/dplex until the cask is accepted upstream into homebrew-cask.
• (Optional follow-up) Submit a PR to upstream Homebrew/homebrew-cask once we've shipped a couple of stable releases.

Reference

• Homebrew Cask cookbook
• Look at any popular Electron app's cask for a template, e.g. Visual Studio Code's cask.

Effort: 4-6 hours including the auto-bump CI.