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

[wgqqqqq]

Summary

Fix the macOS DMG background rendering at 495×300 points inside the configured 660×400 window, leaving white space along the right and bottom and misaligning the artwork with the installer icons.

Change only the PNG physical-resolution metadata from 96 DPI to 72 DPI. Extend the existing packaging test to compare the background's physical dimensions with the Finder window.

Type and Areas

Type: bug fix

Areas: desktop/Tauri, macOS installer, packaging tests

Motivation / Impact

Finder reads the background's physical dimensions rather than its pixel dimensions alone. The existing 660×400 PNG was tagged at 96 DPI, so macOS displayed it at 75% of the intended size. At 72 DPI, macOS reads it as 660×400 points, matching the configured layout.

Verification

• node --test scripts/desktop-tauri-build.test.mjs — all 18 tests passed.
• pnpm run theme:color-audit:all — all 22 selected surfaces passed.
• git diff --check — passed.
• Native macOS AppKit NSImage check: the original asset reads as 495×300 points; the updated asset reads as 660×400 points.
• Binary chunk comparison confirmed that every PNG chunk other than pHYs is byte-for-byte unchanged, including image pixel data.
• A complete DMG rebuild and mounted Finder visual check were not performed. Rust builds were not run because this changes image metadata and packaging assertions only.
• Remote scenarios: not exercised; this change affects local macOS installer presentation and does not change runtime protocols or behavior.

Reviewer Notes

AI-assisted; lightly tested, with focused automated checks and native image-size verification. The regression assertion accounts for PNG's integer pixels-per-metre rounding.

Checklist

• This PR is focused and does not include secrets, temporary prompts, generated scratch files, or unrelated artifacts.
• Relevant verification is recorded above, or skipped checks are explained.
• User-facing strings, docs, and locales are updated where applicable.