Fix Intel Mac binary not found: afterPack copies wrong arch binaries in cross-arch builds

[Copilot]

When building both x64 and arm64 targets on an ARM64 Mac, the afterPack hook used the build machine's arch instead of the target arch, causing the x64 (Intel) app bundle to receive osx-arm64 binaries at the wrong path — leaving osx-x86/scrcpy/adb absent and crashing on launch.

Changes

• scripts/build_optimize.cjs: Replace common.platformArch() with a new archFromContext() helper that maps electron-builder's context.arch enum to the directory naming convention used in electron/resources/extra/:

// electron-builder Arch enum values
const ARCH_X64 = 1;
const ARCH_ARM64 = 3;

const archFromContext = (contextArch) => {
    switch (contextArch) {
        case ARCH_X64:   return "x86";   // osx-x86, win-x86, linux-x86
        case ARCH_ARM64: return "arm64"; // osx-arm64, linux-arm64
        default:         return common.platformArch();
    }
};

afterPack now resolves the correct source directory per target arch, so an ARM64 machine building both slices copies osx-x86 into the x64 bundle and osx-arm64 into the arm64 bundle as intended.