feat(desktop): build Windows arm64 alongside x64

Multi-arch Windows build in one electron-builder pass, with the website
self-healing across the rename. Asset names go from
`Paseo-Setup-${version}.exe` to `Paseo-Setup-${version}-{x64,arm64}.exe`.

The website now reads the actual installer filename off the latest
GitHub release, so legacy single-arch releases keep rendering a single
"Download" pill while dual-arch releases promote to "Intel / x64" +
"ARM64" automatically. No homepage code change needed when ARM ships.

.github/workflows/desktop-release.yml

@@ -355,7 +355,7 @@ jobs:
           PASEO_DESKTOP_SMOKE: "1"
         run: |
           set -euo pipefail
-          build_args=(-- --publish never --win --x64)
+          build_args=(-- --publish never --win --x64 --arm64)
           build_args+=("-c.publish.releaseType=$RELEASE_TYPE")
           build_args+=("-c.publish.channel=$RELEASE_CHANNEL")
           npm run build --workspace="$DESKTOP_WORKSPACE" "${build_args[@]}"