fix(desktop): match DMG background resolution to Finder layout #6127

Job	Run time
Build Impact	50s
Shell Deploy Scripts	13s
CLI Tests (ubuntu-latest)	3m 25s
CLI Tests (windows-latest)	3m 13s
CLI Tests (macos-15)	1m 28s
Frontend Build	3m 25s
Rust Build Check (macos-15)	2m 5s
Rust Build Check (windows-latest)	3m 14s
Rust Build Check (ubuntu-latest)	2m 35s
Impact-selected Linux / Relay Contract	0s
DSH Profile Packaging (windows-latest)	0s
Rust / CLI Validation	6s
	20m 34s

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