feat: Implement a background to the installer dmg (#1662)

## Problem

The macOS DMG installer opens with a plain white background and no visual guidance for drag-to-install.

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## ![CleanShot 2026-04-14 at 21.23.26@2x.png](https://app.graphite.com/user-attachments/assets/e5cb2830-816c-410d-9885-02a6cbd48426.png)

Closes #638

## Changes

1. Add custom background image for the DMG window
2. Configure icon size, positions and window dimensions to match the background layout
3. Place app and Applications shortcut at correct coordinates for the drag-to-install flow

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## How did you test this?

Manually

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Author: charlesvien

apps/code/build/dmg-background.png

@@ -173,6 +173,13 @@ const config: ForgeConfig = {
     new MakerDMG({
       icon: "./build/app-icon.icns",
       format: "ULFO",
+      background: "./build/dmg-background.png",
+      iconSize: 80,
+      window: { size: { width: 540, height: 380 } },
+      contents: (opts) => [
+        { x: 135, y: 225, type: "file", path: opts.appPath },
+        { x: 405, y: 225, type: "link", path: "/Applications" },
+      ],
       ...(shouldSignMacApp && appleCodesignIdentity
         ? {
             "code-sign": {