Add polished configurable installers

[Koshkej]

Summary

• generate consistent DMG, NSIS, and WiX artwork from one app icon and a safe JSON theme
• add a balanced drag-to-Applications DMG layout and Linux AppStream metadata
• create SHA-256 package checksums and package artifacts in the cross-platform CI matrix
• document signing, branding, and per-element text/color customization

Validation

• Python generator/public-readiness tests
• fresh generated app frontend checks
• native macOS build and C++ tests
• real CPack DMG mounted and visually inspected in Finder
• Windows NSIS and Linux DEB packaging covered by the PR matrix