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>

docker-compose.yml

@@ -10,9 +10,22 @@ services:
 #    ports:
 #      - "8088:8000"
     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}}"