v0.51.0: offsite-backup UI (felhom-pbs DR) + Model-A double-nest fix

- Backups page: whole-guest backup shown as real DR — target label "Biztonsági szerver –
  külön hardver (PBS)"; app-data "Távoli mentés" card now reflects the PBS offsite tier
  (guestBackupView.Offsite) instead of "nincs beállítva".
- Model-A double-nest fix: appbackup path helpers take a felhom-data NAMESPACE ROOT (no
  internal felhom-data join); backup.Manager.namespaceRoot/AppNamespaceRoot resolve
  HDD-vs-systemDataPath provenance so a drive-resident app's backups land single-nested
  (<drive>/backups/... on the guest = <drive>/felhom-data/backups/... on the host) instead
  of .../felhom-data/felhom-data/.... Writes, deletion (GetStackBackupData/RemoveStack/
  ProtectedHDDPaths), wipe-warning scan, and export updated coherently; legacy double-nest
  dirs kept protected. New appbackup test asserts no doubled segment.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

controller/internal/appexport/estimate.go

@@ -152,9 +152,11 @@ func diskFree(path string) int64 {
 	return size
 }
 
-// ExportDir returns the exports directory on a drive.
+// ExportDir returns the exports directory on a drive. Model A (slice 10): a registered drive's
+// in-guest mount IS the felhom-data namespace root, so exports/ sits directly under it (no
+// felhom-data segment — avoids the .../felhom-data/felhom-data/... double-nest).
 func ExportDir(drivePath string) string {
-	return filepath.Join(drivePath, "felhom-data", "exports")
+	return filepath.Join(drivePath, "exports")
 }
 
 // humanizeBytes converts bytes to human-readable format.