Fix native selection size badge label

[yvette-carlisle]

Summary

• Update the native macOS selection size badge to show explicit output pixel units, e.g. 1806x898px @2x.
• Keep 1x displays compact by omitting @1x.
• Restore the Rust overlay badge text expectations from the earlier mis-targeted display change; the PNG density/export behavior from Preserve Retina density in screenshot exports #231 is unchanged.

Verification

• cargo make fmt-check
• cargo make check-vstyle
• cargo make check-rust
• cargo make check-swift
• cargo test -p rsnap-overlay selection_size_badge_text_uses_monitor_pixel_dimensions
• git diff --check
• scripts/build_and_run.sh --verify (release/prod staged app launched for manual UI check)