fix(release): switch DMG creation from hdiutil/create-dmg to dmgbuild

[owieth]

Problem

The current DMG creation step uses create-dmg which relies on AppleScript/Finder to style the DMG window. This fails on headless CI runners with:

execution error: Finder got an error: AppleEvent handler failed. (-10000)
Failed running AppleScript
Error: Process completed with exit code 64.

Proposed Solution

Switch to dmgbuild (Python). It produces the same output — custom background, icon positions, window size, hidden labels — without relying on Finder or AppleScript.

1. Replace create-dmg with pip3 install dmgbuild + dmgbuild
2. Add scripts/dmg-settings.py with declarative DMG config
3. Add assets/dmg-background.png (custom background image, 700×400px or 1400×800px retina)

Acceptance Criteria

• scripts/dmg-settings.py committed with window/icon/background settings
• assets/dmg-background.png added
• Release workflow uses dmgbuild instead of create-dmg
• DMG opens with correct background, icon positions, and hidden Finder labels