Build a macOS 14 Intel x86_64 release

[sean1888]

Summary

This PR adds an Intel x86_64 build path for the MacBook Pro 13-inch (2020) configuration (2 GHz quad-core Intel Core i5, macOS Sonoma 14.5).

• Locks the Dayflow app target to x86_64 for Debug and Release.
• Makes the DMG release script default to x86_64.
• Adds an hdiutil fallback when create-dmg is unavailable.
• Adds GitHub Actions coverage using Xcode 16 to build, verify, sign, and package the Intel app.

Verification

GitHub Actions completed successfully: https://github.com/sean1888/Dayflow/actions/runs/32680893547

The generated DMG was mounted and verified locally:

• Main executable: x86_64
• Minimum macOS version: 14.0
• Nested executable architecture checks passed
• Code signature verification passed

Download

A downloadable DMG and SHA-256 checksum are published on the fork branch release:
https://github.com/sean1888/Dayflow/releases/tag/intel-2.1.1

The upstream repository did not grant direct push access, so this PR is submitted from the fork sean1888/Dayflow.

No upstream release or signing credentials are required to review or reproduce the build.