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>

frontend/lib/api.ts

@@ -426,6 +426,11 @@ export class ZFSManagerAPI {
     return response.data
   }
 
+  async getDiskUsage(): Promise<{ filesystems: { filesystem: string; mountpoint: string; total: number; used: number; available: number; capacity: number }[] }> {
+    const response = await this.client.get("/api/system/disk-usage")
+    return response.data
+  }
+
   async getServices(): Promise<{ services: any[] }> {
     const response = await this.client.get("/api/system/services")
     return response.data