build: add universal macOS binary (aarch64 + x86_64) (#90)

Add x86_64-apple-darwin Rust target alongside the existing aarch64 target
and switch Tauri build to --target universal-apple-darwin, producing a single
DMG that runs natively on both Apple Silicon and Intel Macs.

.github/workflows/desktop-release.yml

@@ -42,7 +42,7 @@ jobs:
       - name: Install Rust stable
         uses: dtolnay/rust-toolchain@stable
         with:
-          targets: aarch64-apple-darwin
+          targets: aarch64-apple-darwin,x86_64-apple-darwin
 
       - name: Install JS dependencies
         run: npm ci
@@ -110,4 +110,4 @@ jobs:
           releaseBody: See the assets to download and install this version.
           releaseDraft: false
           prerelease: false
-          args: --target aarch64-apple-darwin
+          args: --target universal-apple-darwin