Switch Linux bundle from appimage to deb

linuxdeploy-plugin-appimage's "continuous" release on GitHub is broken,
causing every AppImage build to fail. The downloaded binary is actually
an HTML error page. Switch to deb format which doesn't depend on
linuxdeploy at all.

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

.github/workflows/desktop-release.yml

@@ -346,7 +346,7 @@ jobs:
           releaseBody: See the assets to download and install this version.
           releaseDraft: ${{ env.RELEASE_DRAFT }}
           prerelease: false
-          args: --bundles appimage
+          args: --bundles deb
 
       - name: Build Linux app (smoke only)
         if: env.IS_SMOKE_TAG == 'true'