fix: add Bun setup to macOS build job for gateway bundling

The beforeBuildCommand in tauri.conf.json uses bun to bundle the
gateway code, so the build-macos job needs bun installed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: createpjf

.github/workflows/build.yml

@@ -59,6 +59,9 @@ jobs:
         with:
           version: 10
 
+      - name: Setup Bun
+        uses: oven-sh/setup-bun@v2
+
       - name: Install Rust
         uses: dtolnay/rust-toolchain@stable
         with: