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/desktop-app/Dockerfile

@@ -3,7 +3,7 @@
 # ===================================================================
 
 # 1. Build Stage (Debian-basiert für Stabilität bei Desktop-Builds)
-FROM gradle:9.2.1-jdk-25-and-25-alpine AS builder
+FROM gradle:9.3.1-jdk-25-and-25-alpine AS builder
 
 WORKDIR /app
 
@@ -55,7 +55,7 @@ RUN set -eu; \
 # ===================================================================
 # Stage 2: Runtime Stage - Ubuntu mit VNC + noVNC
 # ===================================================================
-FROM ubuntu:22.04
+FROM ubuntu:24.04
 
 ENV DEBIAN_FRONTEND=noninteractive
 

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