fix(pc): redesign DMG background and fix horizontal scroll (#245)

- Replace dark background with bright warm-to-cool gradient
  (orange-peach top-left → blue-purple bottom-right)
- Remove arrow and all text from background image
- Fix horizontal scrolling caused by wrong file naming:
  rename background.png (1x, 660x400) + background@2x.png (2x, 1320x800)
  instead of the previous background.png (was 1320x800, treated as 1x)
- Delete obsolete background@1x.png

Co-authored-by: Codex <codex@example.com>

Author: dvlin-dev