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

internal/importer/nfs_test.go

@@ -76,7 +76,7 @@ func TestParseExports(t *testing.T) {
 			if len(got) != tt.wantLen {
 				t.Errorf("parseExports() got %d exports, want %d", len(got), tt.wantLen)
 				for i, e := range got {
-					t.Logf("  export[%d]: path=%q clients=%v options=%q", i, e.Path, e.Clients, e.Options)
+					t.Logf("  export[%d]: path=%q clients=%v fsid=%d", i, e.Path, e.Clients, e.FSID)
 				}
 			}
 		})