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/api/router.go

@@ -561,13 +561,15 @@ func (r *Router) getStackBackupData(w http.ResponseWriter, _ *http.Request, name
 		return
 	}
 
-	// Compute the drive path for this stack (HDD or system data path)
-	var drivePath string
+	// Compute the felhom-data namespace root for this stack (drive-resident apps: the in-guest
+	// mount IS the namespace; SSD-only: <systemDataPath>/felhom-data). Passing the namespace root
+	// (not the raw drive) keeps GetStackBackupData's paths single-nested under Model A.
+	var nsRoot string
 	if r.backupMgr != nil {
-		drivePath = r.backupMgr.GetAppDrivePath(name)
+		nsRoot = r.backupMgr.AppNamespaceRoot(name)
 	}
 
-	resp, err := r.stackMgr.GetStackBackupData(name, drivePath)
+	resp, err := r.stackMgr.GetStackBackupData(name, nsRoot)
 	if err != nil {
 		writeJSON(w, http.StatusNotFound, apiResponse{OK: false, Error: err.Error()})
 		return
@@ -598,9 +600,9 @@ func (r *Router) removeStack(w http.ResponseWriter, req *http.Request, name stri
 	// backup has moved to the host agent; only the app-data DB-dump path is removed here.
 	var backupPaths []string
 	if body.RemoveBackups && r.backupMgr != nil {
-		drivePath := r.backupMgr.GetAppDrivePath(name)
-		if drivePath != "" {
-			backupPaths = append(backupPaths, backup.AppDBDumpPath(drivePath, name))
+		nsRoot := r.backupMgr.AppNamespaceRoot(name)
+		if nsRoot != "" {
+			backupPaths = append(backupPaths, backup.AppDBDumpPath(nsRoot, name))
 		}
 	}