feat: Phase 9 — Offline Detection, API Key Auth, Agent Self-Update

Offline Detection (9.1):
- AgentOfflineDetectorService: BackgroundService, prüft alle 60s
  ob Agents seit >5 min kein Heartbeat hatten → Status=Offline
- IServiceScopeFactory für korrektes Scoped-DI im Singleton
- SignalR-Push AgentStatusChanged bei jeder Offline-Markierung

API Key Auth (9.2):
- ApiKeyMiddleware: prüft X-Api-Key Header gegen Security:ApiKey Config
- Deaktiviert wenn ApiKey leer (Dev-Modus), Swagger/hubs bypassed
- Frontend: getApiKey() aus localStorage, automatisch in allen Requests
- Settings-Modal in Sidebar: API-Key eingeben + maskiert anzeigen

Agent Self-Update (9.3):
- internal/updater/updater.go: CheckForUpdate() + Update()
  Download, SHA256-Verify, Windows Batch-Neustart / Linux Shell-Neustart
- AgentReleasesController: GET /api/v1/agent/releases/latest,
  GET /api/v1/agent/releases/download/{platform}
- AgentReleaseOptions: LatestVersion, ReleasePath, Checksum in appsettings
- executeCommand() erhält cfg *Config statt agentID string
  (für ServerAddress-Ableitung im UpdateAgent-Case)

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

Frontend/src/App.tsx

@@ -1,6 +1,6 @@
 import { useState } from 'react'
 import { QueryClient, QueryClientProvider } from '@tanstack/react-query'
-import { LayoutDashboard, Ticket, Bell, Package, Network, Menu, X } from 'lucide-react'
+import { LayoutDashboard, Ticket, Bell, Package, Network, Menu, X, Settings } from 'lucide-react'
 import { DashboardPage } from './pages/DashboardPage'
 import { AgentDetailPage } from './pages/AgentDetailPage'
 import TicketsPage from './pages/TicketsPage'
@@ -38,6 +38,19 @@ function AppContent() {
   const [page, setPage] = useState<Page>('dashboard')
   const [selectedAgentId, setSelectedAgentId] = useState<string | null>(null)
   const [sidebarOpen, setSidebarOpen] = useState(true)
+  const [settingsOpen, setSettingsOpen] = useState(false)
+  const [apiKeyInput, setApiKeyInput] = useState('')
+
+  const storedKey = localStorage.getItem('nexusrmm_api_key') ?? ''
+  const maskedKey = storedKey.length > 0
+    ? storedKey.substring(0, Math.min(8, storedKey.length)) + '...'
+    : '(nicht gesetzt)'
+
+  function handleSaveApiKey() {
+    localStorage.setItem('nexusrmm_api_key', apiKeyInput)
+    setSettingsOpen(false)
+    setApiKeyInput('')
+  }
 
   function handleSelectAgent(agentId: string) {
     setSelectedAgentId(agentId)
@@ -96,14 +109,73 @@ function AppContent() {
           ))}
         </nav>
 
-        {/* Version */}
-        {sidebarOpen && (
-          <div className="px-4 py-3 text-xs text-muted-foreground border-t border-border">
-            NexusRMM v0.1.0
-          </div>
-        )}
+        {/* Settings + Version */}
+        <div className="border-t border-border px-2 py-2 flex flex-col gap-1">
+          <button
+            onClick={() => {
+              setApiKeyInput(localStorage.getItem('nexusrmm_api_key') ?? '')
+              setSettingsOpen(true)
+            }}
+            className="flex items-center gap-3 px-2 py-2 rounded-md text-sm transition-colors w-full text-left text-muted-foreground hover:text-foreground hover:bg-accent"
+          >
+            <span className="flex-shrink-0"><Settings size={18} /></span>
+            {sidebarOpen && <span>Einstellungen</span>}
+          </button>
+          {sidebarOpen && (
+            <div className="px-2 py-1 text-xs text-muted-foreground">
+              NexusRMM v0.1.0
+            </div>
+          )}
+        </div>
       </aside>
 
+      {/* Settings Modal */}
+      {settingsOpen && (
+        <div className="fixed inset-0 z-50 flex items-center justify-center bg-black/50">
+          <div className="bg-card border border-border rounded-lg shadow-lg w-96 p-6 flex flex-col gap-4">
+            <div className="flex items-center justify-between">
+              <h2 className="text-base font-semibold text-foreground">Einstellungen</h2>
+              <button
+                onClick={() => setSettingsOpen(false)}
+                className="text-muted-foreground hover:text-foreground"
+              >
+                <X size={16} />
+              </button>
+            </div>
+            <div className="flex flex-col gap-2">
+              <label className="text-sm font-medium text-foreground">API-Key</label>
+              <p className="text-xs text-muted-foreground">
+                Aktuell: <span className="font-mono">{maskedKey}</span>
+              </p>
+              <input
+                type="password"
+                value={apiKeyInput}
+                onChange={(e) => setApiKeyInput(e.target.value)}
+                placeholder="API-Key eingeben..."
+                className="w-full px-3 py-2 rounded-md border border-border bg-background text-foreground text-sm placeholder:text-muted-foreground focus:outline-none focus:ring-2 focus:ring-primary"
+              />
+              <p className="text-xs text-muted-foreground">
+                Leer lassen um Authentifizierung zu deaktivieren.
+              </p>
+            </div>
+            <div className="flex justify-end gap-2">
+              <button
+                onClick={() => setSettingsOpen(false)}
+                className="px-4 py-2 text-sm rounded-md border border-border text-muted-foreground hover:text-foreground hover:bg-accent transition-colors"
+              >
+                Abbrechen
+              </button>
+              <button
+                onClick={handleSaveApiKey}
+                className="px-4 py-2 text-sm rounded-md bg-primary text-primary-foreground hover:bg-primary/90 transition-colors"
+              >
+                Speichern
+              </button>
+            </div>
+          </div>
+        </div>
+      )}
+
       {/* Main content */}
       <main className="flex-1 overflow-auto">
         {page === 'dashboard' && (

Frontend/src/api/client.ts

@@ -20,9 +20,18 @@ import type {
 
 const BASE_URL = '/api/v1'
 
+// API-Key aus localStorage lesen (leer = Auth deaktiviert)
+function getApiKey(): string {
+  return localStorage.getItem('nexusrmm_api_key') ?? ''
+}
+
 async function request<T>(path: string, options?: RequestInit): Promise<T> {
   const res = await fetch(`${BASE_URL}${path}`, {
-    headers: { 'Content-Type': 'application/json', ...options?.headers },
+    headers: {
+      'Content-Type': 'application/json',
+      ...(getApiKey() ? { 'X-Api-Key': getApiKey() } : {}),
+      ...options?.headers
+    },
     ...options,
   })
   if (!res.ok) {