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/web/templates/backups.html

@@ -140,10 +140,17 @@
         </div>
     {{end}}
 
+    {{if and .GuestBackup .GuestBackup.Offsite}}
+    <div class="stat-card stat-running">
+        <div class="stat-value">&#10003;</div>
+        <div class="stat-label">Távoli mentés<br><span class="relative-time">külön hardveren (PBS)</span></div>
+    </div>
+    {{else}}
     <div class="stat-card" style="border-left-color: var(--gray);">
         <div class="stat-value" style="background:var(--gray);-webkit-background-clip:text;background-clip:text;">–</div>
         <div class="stat-label">Távoli mentés<br><span class="relative-time">nincs beállítva</span></div>
     </div>
+    {{end}}
 
     <div class="stat-card stat-total">
         <div class="stat-value">