build: macOS builds are now arm64-only, consider universal binaries

[ErikBjare]

Context

Since the CI matrix was updated to use macos-latest (now macos-15-arm64) instead of the previous macos-13 x86_64 runners, all macOS builds (both aw-qt and Tauri) are now arm64-only. This is a notable change from previous releases which were x86_64.

The v0.14.0b1 prerelease is the first release affected.

Impact

• Users on Apple Silicon Macs: no change (arm64 runs natively)
• Users on Intel Macs: binaries won't run (no Rosetta translation for native arm64 builds... actually they should run under Rosetta 2, but it's still a regression from providing native x86_64 binaries)

Options

1. Universal binaries — use lipo to combine x86_64 and arm64 into universal binaries. Requires building on both architectures and merging. More complex but best UX.
2. Add back an x86_64 runner — use macos-13 (the last x86_64 runner) alongside macos-latest and upload both. Simpler but produces two separate downloads. Note: macos-13 will eventually be deprecated.
3. Cross-compile — build x86_64 on the arm64 runner. Possible for Rust (--target x86_64-apple-darwin) but harder for PyInstaller.
4. Do nothing — arm64 binaries run fine on Intel Macs via Rosetta 2. Just fix the filename to say arm64 instead of x86_64 (done in ci: add -tauri suffix to Tauri build release assets #1262).

References

• ci: drop redundant macos-14 from CI matrix #1261 removed the redundant macos-14 runner
• ci: add -tauri suffix to Tauri build release assets #1262 fixes the DMG filename to use actual arch instead of hardcoded x86_64
• Previous releases were built on macos-13 (x86_64)