fix(dmg): preserve image quality and @2x Retina in background padding

[kdroidFilter]

Summary

Fixes the DMG background image rendering issue introduced in v1.5.0 (#26 fix).

The previous padDmgBackgroundForTitleBar implementation had three bugs:

• Renamed output to background-padded.png, breaking electron-builder's @2x auto-detection (transformBackgroundFileIfNeed looks for <name>@2x.<ext> next to the source)
• Ignored @2x Retina variants, so only the 1x image was padded — Retina displays showed blurry/unpadded backgrounds
• Used the source image type for the padded canvas, which produced black padding instead of transparent when the source had no alpha channel (e.g. TYPE_3BYTE_BGR)

Changes

• Preserve the original filename so electron-builder discovers @2x variants
• Explicitly pad @2x files with 2× MACOS_DMG_TITLE_BAR_HEIGHT pixels (56px for Retina)
• Always use TYPE_INT_ARGB + AlphaComposite.Clear for guaranteed transparent padding
• Add 14 unit tests (dimensions, pixel verification, transparency, @2x, idempotency, edge cases)

Closes #163

Test plan

• 14 unit tests pass (DmgBackgroundPaddingTest)
• Pixel-level verification: original colors preserved, padding fully transparent
• Visual inspection of generated images (1x and @2x)
• ktlint + detekt pass without cache
• Gradle cache (up-to-date) works correctly on re-run
• Manual: build a DMG with a background image and verify it displays correctly