Feature: Disk Usage via df im Dashboard (LXC-kompatibel)

- get_disk_usage() in system_info.py via /usr/bin/df -P
- GET /api/system/disk-usage Endpoint
- getDiskUsage() im API-Client
- Dashboard zeigt Disk Usage Karten mit Balken + Total/Used/Free
  (sichtbar auf LXC wo /proc/diskstats keine Blockgeräte liefert)

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

backend/routers/system.py

@@ -168,7 +168,16 @@ async def get_network_traffic():
     return result
 
 
-# ============== DISK I/O ==============
+# ============== DISK USAGE + I/O ==============
+
+@router.get("/disk-usage")
+async def get_disk_usage():
+    """Get filesystem disk usage from df (public)"""
+    result = system_info.get_disk_usage()
+    if "error" in result:
+        raise HTTPException(status_code=500, detail=result["error"])
+    return result
+
 
 @router.get("/diskio")
 async def get_diskio():