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/backup/restore.go

@@ -83,7 +83,7 @@ func (m *Manager) RestoreApp(stackName, snapshotID string) error {
 
 // restoreDockerVolumes populates Docker volumes from tar files in the volume dump directory.
 func (m *Manager) restoreDockerVolumes(stackName, drivePath string) error {
-	dumpDir := AppVolumeDumpPath(drivePath, stackName)
+	dumpDir := AppVolumeDumpPath(m.namespaceRoot(drivePath), stackName)
 	entries, err := os.ReadDir(dumpDir)
 	if err != nil {
 		if os.IsNotExist(err) {