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>

README.md

@@ -63,16 +63,29 @@ The Angular frontend is built for production and packaged into the Spring Boot a
 
 ## Simple Scenario of using
 
-```bash
+```yaml
 services:
   htop-web:
     image: gitea.jambotron.com/liosha84/htop-web:latest
     container_name: htop-web
     restart: unless-stopped
     pid: host
+    read_only: true
+    security_opt:
+      - no-new-privileges:true
+    tmpfs:
+      - /tmp:noexec,nosuid,size=64m
     volumes:
+      - /proc:/host/proc:ro
       - /sys:/host/sys:ro
-      - /:/host/root:ro
+      - /etc/hostname:/host/etc/hostname:ro
+      - /etc/os-release:/host/etc/os-release:ro
+      - /var:/host/var:ro
+      - /home:/host/home:ro
+      - /opt:/host/opt:ro
+      - /tmp:/host/tmp:ro
+      - /srv:/host/srv:ro
+      - /usr:/host/usr:ro
     labels:
       caddy: htop-web.jambotron.com
       caddy.reverse_proxy: "{{upstreams 8080}}"
@@ -84,7 +97,7 @@ networks:
     external: true
 ```
 
-```bash 
+```bash
 docker compose up -d
 ```
 
@@ -93,13 +106,28 @@ docker compose up -d
 
 ### Requirements
 
-- Java 21 and a compatible Node.js/npm installation for local builds.
 - Docker Engine or Docker Desktop.
 - Docker Compose.
 
-### 1. Build the application
+No local Java or Node.js is required — the Dockerfile uses multi-stage build to compile everything inside Docker.
 
-Run from the repository root.
+### 1. Build and start the container
+
+From the repository root, run a single command:
+
+```bash
+docker compose up -d --build
+```
+
+The `--build` flag triggers the multi-stage Docker build that compiles both the Angular frontend and the Spring Boot backend inside Docker.
+
+The default Compose configuration maps host port `8088` to container port `8080`.
+
+Open [http://localhost:8088](http://localhost:8088).
+
+### Alternative: Local Gradle build (requires Java 21 and Node.js)
+
+If you need to build locally (for example, to run tests):
 
 **Linux:**
 
@@ -119,16 +147,6 @@ cd ..
 
 The configured Gradle build builds the frontend and packages its production assets with the backend.
 
-### 2. Build and start the container
-
-```bash
-docker compose up -d --build
-```
-
-The default Compose configuration maps host port `8088` to container port `8080`.
-
-Open [http://localhost:8088](http://localhost:8088).
-
 ### 3. Inspect logs
 
 ```bash