Refactor: Diagnose-Tab in Zentrale gemerged + Zentrale neu strukturiert

Diagnose war groesstenteils Dublette (Metriken/Temps schon in Zentrale; Logs/Restart/
Updates im Pflege-Drawer). Tab entfernt, zwei einzigartige Teile umverteilt.

- Zentrale neu in 3 Zonen: Live-Telemetrie (System-Status + Token-Durchsatz nebeneinander
  statt gestapelt), Stack-Status (Aktive Modelle / Rollen / Dienste), Betrieb & Wissen
  (Updates / Hermes / Gedaechtnis).
- Neue ServicesCard (Dienste-Health aus Diagnose) auf der Zentrale.
- TokenStatsCard ('Effizienz & Ersparnis') in TokenPerformanceCard gemerged (Input/Output
  + gespart) -> eine Karte weniger, keine Dublette.
- Backup/Snapshot in den System-Wartung-Drawer verschoben.
- nav.ts/App.tsx: 'system'-View entfernt (6 statt 7 Tabs); SystemView.tsx geloescht.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -4,7 +4,6 @@ import { NAV, type ViewId } from "@/nav"
 import { CommandPalette } from "@/components/CommandPalette"
 import { DashboardView } from "@/views/DashboardView"
 import { ModelsView } from "@/views/ModelsView"
-import { SystemView } from "@/views/SystemView"
 import { ConnectView } from "@/views/ConnectView"
 import { MemoryView } from "@/views/MemoryView"
 import { AgentView } from "@/views/AgentView"
@@ -176,12 +175,11 @@ export default function App() {
         <main className="flex-1 overflow-y-auto p-6 scrollbar-thin">
           {view === "dashboard" && <DashboardView />}
           {view === "models" && <ModelsView />}
-          {view === "system" && <SystemView />}
           {view === "connect" && <ConnectView />}
           {view === "memory" && <MemoryView />}
           {view === "agent" && <AgentView />}
           {view === "guide" && <GuideView />}
-          {!["dashboard", "models", "system", "connect", "memory", "agent", "guide"].includes(view) && (
+          {!["dashboard", "models", "connect", "memory", "agent", "guide"].includes(view) && (
             <Placeholder title={active.label} hint={active.hint} />
           )}
         </main>