Desktop: add a branded macOS DMG installer window

[Brechard]

Summary

The macOS DMG currently uses the default installer presentation. This issue proposes supporting a branded Finder drag-and-drop window for the desktop installer.

Proof of concept

I prepared a working proof of concept in the accompanying PR. It adds a small, configurable DMG visual system with three sample backgrounds and keeps the asset selection out of the packaging implementation.

The artwork in this PR is intentionally placeholder/proof-of-concept material. The project team can replace it with any official T3 Code artwork, copy, typography, or brand treatment before merging.

Why this is useful

A polished DMG window gives first-time desktop users a clearer installation path and makes the installer feel like part of the product rather than a generic packaging artifact.

Scope of the proposed change

• Configure the DMG Finder window size and application/Applications placement.
• Allow the background asset to be selected through a build option.
• Keep the visual asset replaceable without changing packaging logic.
• Include focused tests for the generated electron-builder configuration.

The custom image is implemented using electron-builder's supported DMG background configuration; Finder will keep its normal DMG metadata files hidden when hidden files are not enabled.