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>

backend/build.gradle

@@ -66,7 +66,11 @@ tasks.register('buildFrontend', Exec) {
 	}
 }
 
+// Skip frontend build tasks when building inside Docker (frontend already built in Stage 1)
+def skipFrontend = project.hasProperty('skipFrontend') || System.getenv('SKIP_FRONTEND') != null
+
 tasks.register('copyFrontend', Copy) {
+	onlyIf { !skipFrontend }
 	dependsOn 'buildFrontend'
 	outputs.upToDateWhen { false }