chore: add custom dmg background #71

Job	Run time
build-linux	7m 34s
build-windows	26m 13s
build-macos-slices (macos-15-intel, x86_64-apple-darwin, intel)	38m 29s
build-macos-slices (macos-latest, aarch64-apple-darwin, arm)	15m 37s
build-macos-universal	1m 22s
	1h 29m 15s

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