Refactor: Java-Klassen aus Services entfernt + kritische Bugs gefixt

- AuthService, SystemInfo, IdentitiesManager Klassen → Modul-Funktionen
- grp.getall() → grp.getgrall() (Bug: Methode existierte nie)
- open('/proc/loadavg') ohne context manager gefixt (File-Handle-Leak)
- rx_packets/tx_packets null-check im Frontend (toLocaleString auf undefined)
- PoolCard onClick: /pools/{name} → /zfs (Route existierte nicht, löste Seitenreload aus)
- Alle Router-Imports auf Modul-Aliase umgestellt

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

backend/routers/identities.py

@@ -7,8 +7,8 @@ from fastapi.security import HTTPBearer, HTTPAuthorizationCredentials
 from pydantic import BaseModel
 from typing import Optional
 
-from services.identities import identities_manager
-from services.auth import auth_service
+from services import identities as identities_manager
+from services import auth as auth_service
 
 router = APIRouter(prefix="/api/identities", tags=["identities"])
 security = HTTPBearer()