feat: embed AppImage update information for delta updates

Add a top-level publish config in electron-builder.json5 pointing to
GitHub Releases. This embeds the update information URL in the AppImage
header, enabling tools like AppImageUpdate, AppImageLauncher, and
AppManager to perform delta updates instead of full re-downloads.

Also update the Linux build workflow to upload the generated .zsync file
alongside the .AppImage artifact.

Fixes #219

.github/workflows/build.yml

@@ -94,6 +94,7 @@ jobs:
         uses: actions/upload-artifact@v4
         with:
           name: linux-installer
-          path: release/**/*.AppImage
+          path: |
+            release/**/*.AppImage
+            release/**/*.zsync
           retention-days: 30
-