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.

config/docker/nginx/web-app/Dockerfile

@@ -5,7 +5,7 @@
 # ===================================================================
 
 # === GLOBAL ARGS ===
-ARG GRADLE_VERSION=9.2.1
+ARG GRADLE_VERSION=9.3.1
 ARG JAVA_VERSION=25
 ARG NGINX_IMAGE_TAG=1.28-alpine
 ARG VERSION=1.0.0-SNAPSHOT