feat: Phase 8 — Network Discovery + Windows Dev-Setup-Skripte

Network Discovery:
- Go Agent: internal/scanner/scanner.go mit TCP-Sweep (Port 445/80/22/443),
  ARP-Tabellen-Parser (Windows: arp -a, Linux: /proc/net/arp), Reverse-DNS,
  50 gleichzeitige Goroutines mit Semaphore
- Go Agent main.go: COMMAND_TYPE_NETWORK_SCAN Case → scanner.Scan() → JSON stdout
- Backend: NetworkDevice Model (Id, AgentId, IpAddress, MacAddress, Hostname,
  Vendor, IsManaged, FirstSeen, LastSeen)
- Backend: EF Migration AddNetworkDevices + Index auf IpAddress + MacAddress
- Backend: NetworkDevicesController GET /api/v1/network-devices + DELETE /{id}
- Backend: AgentGrpcService.ProcessNetworkScanResultAsync — upsert via MAC,
  IsManaged=true wenn IP einem bekannten Agent entspricht
- Frontend: NetworkPage.tsx mit Scan-Panel, Device-Tabelle, Filter, Delete
- Frontend: App.tsx — 'Netzwerk' Nav-Eintrag mit Network Icon

Windows Dev-Setup:
- dev-start.ps1 — Startet Docker/Postgres, EF-Migrationen, Backend+Frontend
  in separaten PowerShell-Fenstern; Voraussetzungen-Check (docker/dotnet/node/go)
- dev-stop.ps1 — Stoppt alle NexusRMM-Prozesse + PostgreSQL Container
- build-agent.ps1 — Baut nexus-agent.exe (Windows) + optional nexus-agent-linux

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Agent/cmd/agent/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"encoding/json"
 	"fmt"
 	"log"
 	"os"
@@ -16,6 +17,7 @@ import (
 	"nexusrmm.local/agent/internal/deployer"
 	"nexusrmm.local/agent/internal/executor"
 	"nexusrmm.local/agent/internal/meshagent"
+	"nexusrmm.local/agent/internal/scanner"
 	pb "nexusrmm.local/agent/pkg/proto"
 )
 
@@ -142,6 +144,21 @@ func executeCommand(ctx context.Context, client *connection.GrpcClient, agentID
 		result = deployer.Install(ctx, cmd.Payload)
 	case pb.CommandType_COMMAND_TYPE_UNINSTALL_SOFTWARE:
 		result = deployer.Uninstall(ctx, cmd.Payload)
+	case pb.CommandType_COMMAND_TYPE_NETWORK_SCAN:
+		var params struct {
+			Subnet string `json:"subnet"`
+		}
+		_ = json.Unmarshal([]byte(cmd.Payload), &params)
+		devices, err := scanner.Scan(ctx, params.Subnet)
+		if err != nil {
+			result = &executor.Result{ExitCode: 1, Stderr: err.Error()}
+		} else {
+			result = &executor.Result{
+				ExitCode: 0,
+				Stdout:   scanner.ToJSON(devices),
+				Success:  true,
+			}
+		}
 	default:
 		result = &executor.Result{ExitCode: -1, Stderr: fmt.Sprintf("unknown command type: %v", cmd.Type)}
 	}