Refactor: TanStack-Query-Daten-Layer + useDialog (Phase 3b)

- @tanstack/react-query (v5) als zentraler Daten-Layer; QueryClientProvider
  in main.tsx (retry 1, kein refetchOnWindowFocus, staleTime 5s).
- lib/queries.ts: Domänen-Hooks (useHealth/useSystemStatus/useServices/
  useModels/useRouting/useJobs/useTokenStats/useAgentStatus/useUpdates/
  useDiscover/useConnect/useMemory) + zentrale Query-Keys (qk) + invalidate.
  Gleicher Key = eine Anfrage über alle Views (Dedup), einheitliches Polling.
- lib/useDialog.tsx: ein Hook für Alert/Confirm/Prompt statt 5x dupliziertem
  Dialog-State + showAlert/showConfirm.
- api.ts: TokenStats + ModelsResp typisiert.
- Migriert auf Hooks/useDialog: App, SystemView, AgentView, ConnectView,
  MemoryView (manuelles useEffect+setInterval entfernt; Mutationen
  invalidieren gezielt die Query-Keys).

Verifiziert: tsc grün, Build grün, alle migrierten Views ohne Konsolenfehler,
Live-Daten (CPU/RAM/Dienste) rendern korrekt.

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

frontend/src/App.tsx

@@ -11,30 +11,17 @@ import { AgentView } from "@/views/AgentView"
 import { GuideView } from "@/views/GuideView"
 import { Placeholder } from "@/views/Placeholder"
 import { SystemDrawer } from "@/components/SystemDrawer"
-import { api, type Health, type SystemStatus } from "@/lib/api"
+import { useHealth, useSystemStatus } from "@/lib/queries"
 import { cn } from "@/lib/utils"
 
 export default function App() {
   const [view, setView] = useState<ViewId>("dashboard")
-  const [health, setHealth] = useState<Health | null>(null)
   const [sidebarCollapsed, setSidebarCollapsed] = useState(() => localStorage.getItem("mc_sidebar_collapsed") === "true")
-  const [sysStatus, setSysStatus] = useState<SystemStatus | null>(null)
   const [drawerOpen, setDrawerOpen] = useState(false)
   const [drawerTab, setDrawerTab] = useState<"maintenance" | "logs">("maintenance")
 
-  useEffect(() => {
-    const load = () => api<Health>("/api/health").then(setHealth).catch(() => setHealth(null))
-    load()
-    const t = setInterval(load, 10000)
-    return () => clearInterval(t)
-  }, [])
-
-  useEffect(() => {
-    const loadSys = () => api<SystemStatus>("/api/system/status").then(setSysStatus).catch(() => {})
-    loadSys()
-    const t = setInterval(loadSys, 20000)
-    return () => clearInterval(t)
-  }, [])
+  const { data: health } = useHealth()
+  const { data: sysStatus } = useSystemStatus(20_000)
 
   useEffect(() => {
     document.documentElement.classList.add("dark")