feat: add storage info to settings page and improve NFS export handling

Backend:
- Add source/used_by/available/error fields to diskUsage in system status
- collectDiskUsage() now reads /proc/mounts, samba shares and NFS exports from DB
- Paths are deduplicated; shared paths list all shares/exports using them
- syscall.Statfs errors surface as available=false with user-facing error
- collectServiceStatus made a method of Server (receiver consistency)

Frontend:
- Settings page now shows two cards: mount points and shared resources
- Each path shows source badge (Sistema/Mount/SMB/NFS), used_by chips, progress bar
- Unavailable paths show amber warning instead of progress bar
- DiskUsage interface updated with new fields
- NFSExport interface updated with structured fields (fsid, async, etc)
- NFS page updated to use new export fields

web/src/api.ts

@@ -13,7 +13,12 @@ export interface NFSExport {
   id: number;
   path: string;
   clients: string[];
-  options: string;
+  read_only: boolean;
+  async: boolean;
+  root_squash: boolean;
+  subtree_check: boolean;
+  fsid: number;
+  advanced: string;
 }
 
 export interface User {
@@ -43,6 +48,10 @@ export interface DiskUsage {
   free_bytes: number;
   used_bytes: number;
   used_percent: number;
+  source: "system" | "mount" | "samba" | "nfs";
+  used_by?: string[];
+  available: boolean;
+  error?: string;
 }
 
 export interface ServiceStatus {