Create portableweb/homebrew-tap for the Tauri viewer cask

[omscse]

Create a Homebrew tap so users can install the desktop viewer on macOS with:

brew tap portableweb/tap
brew install --cask portableweb-viewer

Steps:

1. Create repo portableweb/homebrew-tap
2. Add a Casks/portableweb-viewer.rb cask formula pointing to the signed .dmg
3. Document installation in viewer README

Depends on: sign and notarize the macOS build first (see related issue).