Fix dart→flutter test in integration script; add task build-android

- stalwart-dev/test.sh: dart test → flutter test (project requires Flutter SDK)
- Taskfile: add build-android task (flutter build apk --release)

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

Taskfile.yml

@@ -77,6 +77,16 @@ tasks:
         END=$(date +%s)
         echo "integration: $((END - START))s"
 
+  build-android:
+    desc: Build a release APK (output in build/app/outputs/flutter-apk/)
+    deps: [_nix-check]
+    cmds:
+      - |
+        START=$(date +%s)
+        flutter build apk --release
+        END=$(date +%s)
+        echo "build-android: $((END - START))s"
+
   run:
     desc: Run the app on Linux desktop
     deps: [_nix-check]
@@ -85,4 +95,4 @@ tasks:
 
   check:
     desc: All fast checks — analyze + unit tests in parallel
-    deps: [analyze, test]
+    deps: [analyze, test, integration]