test(desktop): add Compose UI smoke test + release .deb launch CI

Fixes #2819 — v1.08.0 .deb crashed on Ubuntu with ManagementFactory
error because the build had no jlink modules() declaration. Current
main already has the fix; this PR adds CI to prevent regressions.

- Add compose.desktop.uiTestJUnit4 dependency
- DesktopLaunchSmokeTest: renders LoginScreen, asserts title text
- build.yml: xvfb for Linux leg so UI test runs on every PR
- smoke-test-desktop.yml: builds release .deb, installs it, launches
  under xvfb, verifies process stays alive 10s

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/build.yml

@@ -155,8 +155,20 @@ jobs:
               ;;
           esac
 
+      # Compose UI smoke test (DesktopLaunchSmokeTest) uses Skiko which needs
+      # a display server on Linux. xvfb provides a virtual framebuffer.
+      - name: Install xvfb (Linux)
+        if: runner.os == 'Linux'
+        run: sudo apt-get update && sudo apt-get install -y xvfb
+
       - name: Test + Build Desktop (gradle)
-        run: ./gradlew :quartz:jvmTest :commons:jvmTest :nestsClient:jvmTest :cli:test :desktopApp:test :desktopApp:${{ matrix.desktop-task }}
+        run: |
+          CMD="./gradlew :quartz:jvmTest :commons:jvmTest :nestsClient:jvmTest :cli:test :desktopApp:test :desktopApp:${{ matrix.desktop-task }}"
+          if [ "${{ runner.os }}" = "Linux" ]; then
+            xvfb-run --auto-servernum $CMD
+          else
+            $CMD
+          fi
 
       # jpackage pins libicu to the build host's version (libicu74 on
       # ubuntu-24.04). Rewrite the .deb so testers on other Debian/Ubuntu