feat: init pc-monitor project

- Client: Go-based Windows hardware monitoring (CPU, GPU, memory, disk, network, power)
- Server: Go + Gin + SQLite backend with REST API
- Frontend: Vue 3 + Element Plus dashboard
- Docker deployment support
- Windows service installation script

server/service/device.go

@@ -0,0 +1,73 @@
+package service
+
+import (
+	"crypto/sha256"
+	"fmt"
+	"pc-monitor-server/model"
+	"pc-monitor-server/repository"
+	"time"
+)
+
+type DeviceService struct {
+	repo *repository.DeviceRepository
+}
+
+func NewDeviceService(repo *repository.DeviceRepository) *DeviceService {
+	return &DeviceService{repo: repo}
+}
+
+func (s *DeviceService) Register(hostname, osName, ip string) (*model.Device, error) {
+	id := generateDeviceID(hostname, ip)
+	device, err := s.repo.GetByID(id)
+	if err == nil {
+		device.LastReportAt = time.Now()
+		device.Status = "online"
+		s.repo.Update(device)
+		return device, nil
+	}
+
+	device = &model.Device{
+		ID:           id,
+		Hostname:     hostname,
+		OS:           osName,
+		IP:           ip,
+		RegisteredAt: time.Now(),
+		LastReportAt: time.Now(),
+		Status:       "online",
+	}
+	if err := s.repo.Create(device); err != nil {
+		return nil, err
+	}
+	return device, nil
+}
+
+func (s *DeviceService) GetAll() ([]model.Device, error) {
+	return s.repo.GetAll()
+}
+
+func (s *DeviceService) GetByID(id string) (*model.Device, error) {
+	return s.repo.GetByID(id)
+}
+
+func (s *DeviceService) Delete(id string) error {
+	return s.repo.Delete(id)
+}
+
+func (s *DeviceService) Heartbeat(id string) error {
+	device, err := s.repo.GetByID(id)
+	if err != nil {
+		return err
+	}
+	device.LastReportAt = time.Now()
+	device.Status = "online"
+	return s.repo.Update(device)
+}
+
+func (s *DeviceService) CheckOffline() {
+	s.repo.MarkOffline(2 * time.Minute)
+}
+
+func generateDeviceID(hostname, ip string) string {
+	hash := sha256.Sum256([]byte(fmt.Sprintf("%s-%s", hostname, ip)))
+	return fmt.Sprintf("%x", hash[:8])
+}