Linux/Proxmox Agent + Plattform-Updater + Proxmox Frontend

- agent-linux/: Kompletter Linux/Proxmox RMM Agent
  - Collectors: CPU, Memory, Disk, Network, Services, Updates, Proxmox (VMs, Container, Storage, ZFS), Backups
  - Backup Collector: Jobs nach Node gefiltert (Cluster-aware)
  - WebSocket: Tunnel-Support, writeMux fuer stabile Verbindungen
  - Systemd Service Template

- updater/: Plattform-unabhaengiger Updater (FreeBSD + Linux)
  - runtime.GOOS erkennt Plattform automatisch
  - FreeBSD: /usr/local/rmm/, service rmm_agent
  - Linux: /usr/local/bin/, /etc/rmm/, systemctl rmm-agent
  - Firmware-Download mit ?platform= Parameter

- frontend/: Proxmox-Bereich
  - ProxmoxServers.jsx: Eigene Seite fuer PVE Server mit VM/CT Counts
  - ProxmoxPanel.jsx: Detail-Panel mit Uebersicht, VMs, Container, Storage, ZFS, Tunnel, Dienste, Updates, Backups
  - Agents.jsx: Filtert Linux-Agents raus (nur OPNsense)
  - Sidebar: Proxmox Navigationspunkt
  - Installationsanleitung mit rmm-agent-linux + rmm-updater-linux

- backend: Platform-Feld fuer Agents, Firmware multi-platform

backend/api/handlers.go

@@ -130,6 +130,11 @@ func (h *Handler) registerAgent(w http.ResponseWriter, r *http.Request) {
 		agentID = generateID()
 	}
 
+	platform := req.Platform
+	if platform == "" {
+		platform = "freebsd" // Default fuer bestehende OPNsense Agents
+	}
+
 	agent := &models.Agent{
 		ID:              agentID,
 		Name:            req.Name,
@@ -137,6 +142,7 @@ func (h *Handler) registerAgent(w http.ResponseWriter, r *http.Request) {
 		IP:              req.IP,
 		OPNsenseVersion: req.OPNsenseVersion,
 		AgentVersion:    req.AgentVersion,
+		Platform:        platform,
 		RegisteredAt:    time.Now().UTC(),
 	}