feat(ci): restore x86_64 macOS builds via macos-15-intel runner

[ErikBjare]

What

Adds macos-15-intel to both the Qt and Tauri build matrices, restoring Intel macOS artifacts dropped when the matrix moved to macos-latest (arm64). Closes #1263 (option 2).

Why this is viable again

GitHub retired the free macos-13 x86_64 image, but introduced macos-15-intel as its designated standard-image successor specifically due to OSS demand — see actions/runner-images#13045. It is available until August 2027, and is the last x86_64 image GitHub Actions will ever offer; after that the options are universal binaries built on arm64 or dropping Intel for good. Since arm64 binaries do not run on Intel via Rosetta (translation is x86_64→arm64 only), this runner is currently the only practical way to ship Intel-capable artifacts.

Why it should just work

• With MACOSX_DEPLOYMENT_TARGET=12.0 (fix(ci): bump MACOSX_DEPLOYMENT_TARGET from 10.9 to 12.0 #1363), x86_64 artifacts support Intel Macs back to ~2014–15 (Monterey); the Swift helper verifiably compiles for x86_64-apple-macosx12.0
• Artifact names already use uname -m (ci: add -tauri suffix to Tauri build release assets #1262) → ...-macos-x86_64.dmg
• All macOS steps key off runner.os == 'macOS', none off macos-latest
• Signing/notarization run per-matrix-job on separate runners; the Developer ID cert is arch-independent

Sequencing

Suggest merging after the v0.14.0b2 re-release is cut, so the beta isn't blocked if the Intel job surfaces surprises. This PR's own CI exercises the full x86_64 build path (unsigned).

[ErikBjare]

@greptileai review

[greptile-apps]

Greptile Summary

This PR restores Intel macOS release builds. The main changes are:

• Adds macos-15-intel to the Qt build matrix.
• Adds macos-15-intel to the Tauri build matrix.
• Reuses architecture-specific artifact names and runner-specific caches.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.
• Cache and upload names include the matrix runner.
• Final DMG names include the machine architecture, preventing ARM and Intel collisions.

Important Files Changed

Filename	Overview
.github/workflows/release.yml	Adds Intel macOS jobs to both release matrices; existing cache keys and artifact names keep ARM and Intel outputs separate.

_{Reviews (1): Last reviewed commit: "feat(ci): restore x86_64 macOS builds vi..." | Re-trigger Greptile}

[greptile-apps]

Greptile Summary

This PR restores Intel macOS release builds. The main changes are:

• Adds macos-15-intel to the Qt build matrix.
• Adds macos-15-intel to the Tauri build matrix.
• Produces architecture-specific x86_64 artifacts through the existing packaging flow.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.
• Both jobs use the existing macOS build, test, signing, and packaging paths.
• Matrix-scoped caches and architecture-specific filenames avoid collisions between Intel and ARM64 outputs.

Important Files Changed

Filename	Overview
.github/workflows/release.yml	Adds the Intel macOS runner to both release matrices; existing cache keys, artifact names, and package filenames keep Intel and ARM64 outputs separate.

_{Reviews (2): Last reviewed commit: "feat(ci): restore x86_64 macOS builds vi..." | Re-trigger Greptile}