feat(mac): brand DMG installer layout

[wgqqqqq]

Summary

• add the supplied BitFun installer background to macOS DMGs
• place BitFun and Applications on the two sides of the arrow
• keep Finder DMG aesthetics enabled in GitHub Actions instead of passing Tauri's CI skip flag

Verification

• node --test scripts/desktop-tauri-build.test.mjs (11 passed)
• local ARM64 DMG preview generated and checksum-verified
• local App bundle remains notarized and Gatekeeper accepted

Remote scenarios

Not applicable. This changes only macOS packaging presentation and does not alter runtime remote scenarios.

AI assistance

AI-assisted; focused packaging tests passed.