Add branded macOS installer welcome and background

🤔 Why?

• Make the macOS installer more professional and informative by displaying the product name and version during installation and presenting a branded welcome screen.

🔧 What changed

• Added assets/installer/distribution.xml (installer manifest used by productbuild) with an __ART_VERSION__ placeholder.
• Moved the welcome RTF to assets/installer/Welcome.rtf; CI injects the real version into this file during the build.
• Added scripts/gen_installer_background.py to generate a simple polished background image and produced assets/images/installer-background.png.
• Updated the macOS packaging step in .github/workflows/ci.yml to embed the .icns into the app bundle, copy installer resources from assets/, substitute ART_VERSION into the manifest and welcome RTF, and call productbuild --distribution so the installer UI shows the title, background, and welcome.
• Included a postinstall script that writes Applications/MkPFS.app/Contents/Resources/VERSION.txt with the installed version.

🧪 How to test

• On macOS: build the app bundle with the same PyInstaller invocation used in CI, then run the packaging steps from the workflow to produce the .pkg.
• Open the produced .pkg: the installer title should read MkPFS <version> and show the welcome screen and background.
• After install, check /Applications/MkPFS.app/Contents/Resources/VERSION.txt for the version string.

💬 Notes for reviewers

• The installer intentionally omits a license screen (requested change).
• The background image is a generated placeholder; we plan to replace it with a higher-quality design in a follow-up.

Closes: none