quick smoketest of macos

.github/workflows/macos.yml

@@ -33,6 +33,25 @@ jobs:
                   flutter pub run build_runner build
                   flutter build macos --dart-define=GIT_COMMIT=${{ github.sha }}
 
+            # Only checks it launches and stays open, since there's no one to try it
+            - name: Smoke test
+              run: |
+                  open build/macos/Build/Products/Release/Nexus.app
+                  sleep 20
+                  screencapture -x screenshot.png || true
+                  if ! pgrep -if "Nexus.app/Contents/MacOS"; then
+                      echo "Nexus exited during startup"
+                      exit 1
+                  fi
+
+            - name: Upload screenshot
+              if: always()
+              uses: actions/upload-artifact@v6
+              with:
+                  name: macos-screenshot
+                  path: screenshot.png
+                  if-no-files-found: ignore
+
             - name: Create DMG
               id: create-dmg
               uses: L-Super/create-dmg-actions@28511e988b13ca34096d439159a66d468b8e724f