chore(infra+frontend): upgrade Gradle to 9.3.1 and fix KMP plugin conflicts in Docker builds

- Updated Gradle version in `.env`, Dockerfiles, and wrapper to 9.3.1.
- Replaced alias-based application of `kotlinMultiplatform` plugin with direct `id` usage in subprojects to resolve "Plugin loaded multiple times" error.
- Applied centralized plugin management and Gradle daemon optimizations to improve Docker build stability and address KMP classloading issues.

dc-gui.yaml

@@ -11,7 +11,7 @@ services:
       context: . # Wichtig: Root Context für Monorepo Zugriff
       dockerfile: config/docker/caddy/web-app/Dockerfile
       args:
-        GRADLE_VERSION: "${DOCKER_GRADLE_VERSION:-9.2.1}"
+        GRADLE_VERSION: "${DOCKER_GRADLE_VERSION:-9.3.1}"
         JAVA_VERSION: "${DOCKER_JAVA_VERSION:-25}"
         # Frontend spezifisch:
         CADDY_VERSION: "${DOCKER_CADDY_VERSION:-2.9-alpine}"