feat(dmg): add a guided macOS installer layout

[Colafornia]

Summary

The macOS installer is one of the user's first impressions of Maka. This PR replaces the default DMG presentation with a branded, spacious drag-to-Applications layout.

The new layout adds clear installation guidance, larger icons, and standard and Retina backgrounds. DMG signing and update behavior remain unchanged.

Before and after

Before	After

Verification

• The DMG background regression test fails against the pre-change baseline and passes on this branch.
• Build and full-workspace typecheck passed.
• Biome format and lint passed on all tracked files.
• Release contracts: 114 passed, 0 failed.
• Desktop tests: 1,543 passed, 0 failed.
• Desktop e2e: 66 passed, 1 skipped.
• Browser WebContentsView smoke: 17/17 passed.
• Alignment audit: all fixtures clean.
• The locally built DMG passed checksum verification and mounted with the expected app, Applications link, and Retina background.

Not run: the complete signed and notarized release workflow.

AI use

Select exactly one:

• No generative tool made a substantive contribution
• Generative tooling made a substantive contribution

Tool(s) and scope: Codex reviewed the change, ran local verification, formatted one assertion, and drafted this PR description.

Checklist

• Tests cover the change and fail without it
• Lint, format, typecheck and the affected suites pass locally

Does this PR entail a change in behavior?

• Yes — described under Summary above
• No