R-280: attach list from mounted-but-unregistered filesystems; two-clicks promise made conditional

After a reinstall the data drive could not be re-attached through any dashboard
route: both candidate lists came from the agent's unclaimed-disk scan, and the
rebuilt box's drives are claimed. The restore page said it was two clicks while
pointing at an empty picker.

The attach list now also carries the controller's own mounted-but-unregistered
filesystems. initialize is untouched, so the format wizard's system/backup
protection is unchanged. The 'two clicks' sentence is conditional on the picker
being non-empty, and says something true and actionable when it is not.

controller/internal/web/templates/backups_restore.html

@@ -71,12 +71,24 @@
          repository the whole time. Installed-ness is a property OF a row, never a filter on it. -->
     <!-- R-252: the precondition a rebuilt box fails, said BEFORE the customer presses a button that
          would refuse. Rendered only when it is true — a healthy box sees nothing new here. -->
+    <!-- R-280: the „két kattintás" promise is conditional on the picker it points at being non-empty.
+         It was printed unconditionally, and on a rebuilt box the picker had nothing in it — the
+         sentence sent the customer to an empty page and the wall had no way past it. The false branch
+         says what is true and names a route, rather than promising a click that does not exist. -->
     {{if .NoRestoreDestination}}
     <p class="form-hint" style="border-left:2px solid var(--amber);padding-left:.75rem">
         <strong>Előbb csatold vissza az adatmeghajtót.</strong> A mentéseid megvannak, és a meghajtók is
         megvannak — újratelepítés után viszont a gép még nem ismeri őket, ezért most nincs hová
-        visszaállítani. Ez két kattintás: <a href="/storage" style="color:var(--blue)">Tárhely →
+        visszaállítani.
+        {{if .HasAttachDestination}}
+        Ez két kattintás: <a href="/storage" style="color:var(--blue)">Tárhely →
         Meghajtók</a>, „Meglévő meghajtó csatolása". Utána gyere vissza ide.
+        {{else}}
+        Csatolható meghajtót viszont most nem látunk ezen a gépen, ezért ezt innen nem tudod
+        elindítani. Ha a meghajtó be van kötve, de nincs csatlakoztatva, az üzemeltető tudja
+        csatlakoztatni — jelezd neki. A <a href="/storage" style="color:var(--blue)">Tárhely →
+        Meghajtók</a> oldalon látod, mit ismer most a gép.
+        {{end}}
     </p>
     {{end}}
     {{if eq .OffsiteStoreState "unreadable"}}