Major improvements: multi-stage Docker, security hardening, host metrics, tests

- Multi-stage Docker build: Dockerfile compiles frontend (Node.js) and
  backend (Gradle) inside Docker; no local Java/Node.js required
- Security hardening: replaced full host root mount with specific
  directory mounts (/proc, /sys, /etc, /var, /home, /opt, /tmp, /srv, /usr)
- Host metrics consistency: LinuxMemoryService and CpuDetailsService now
  read from /host/proc instead of container OSHI
- Concurrency: replaced synchronized with AtomicReference in 3 services
- Global error handling: added @RestControllerAdvice for JSON error responses
- Unit tests: added 32 tests for /proc parsers, CPU usage, process uptime
- Docker security: non-root user, read-only FS, no-new-privileges, healthcheck
- Debug logging: added log.debug() in silent catch blocks
- Documentation: updated README, INSTALLATION, DEVELOPMENT, CONFIGURATION,
  ARCHITECTURE, CHANGELOG

Co-authored-by: Junie <junie@jetbrains.com>

Dockerfile

@@ -1,6 +1,34 @@
 
+# Stage 1: Build frontend with Node.js
+FROM node:22-alpine AS frontend
+WORKDIR /frontend
+COPY frontend/package.json frontend/package-lock.json ./
+RUN npm ci
+COPY frontend/ ./
+RUN npm run build -- --configuration production
+
+# Stage 2: Build backend with Gradle
+FROM eclipse-temurin:21-jdk AS builder
+WORKDIR /app
+COPY backend/build.gradle backend/settings.gradle ./
+COPY backend/gradlew ./
+COPY backend/gradle/ gradle/
+COPY backend/src/ src/
+# Copy built frontend assets into Spring static resources
+COPY --from=frontend /frontend/dist/frontend/browser src/main/resources/static/
+RUN chmod +x gradlew && SKIP_FRONTEND=true ./gradlew bootJar --no-daemon
+
+# Stage 3: Runtime image
 FROM eclipse-temurin:21-jre
 WORKDIR /app
-COPY backend/build/libs/backend-0.0.1-SNAPSHOT.jar app.jar
+COPY --from=builder /app/build/libs/backend-0.0.1-SNAPSHOT.jar app.jar
+
+RUN adduser --disabled-password --no-create-home appuser && \
+    chown appuser:appuser /app
+USER appuser
+
 EXPOSE 8080
+HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
+    CMD wget -q --spider http://localhost:8080/api/system/metrics || exit 1
+
 ENTRYPOINT ["java", "-jar", "app.jar"]