v0.15.0: Attach existing drive wizard (bind mount, no format)

New Settings wizard to attach drives with existing filesystems without
formatting. Mounts partition at staging path, lets user browse and pick
a subfolder, then bind-mounts it at /mnt/<name> with fstab entries.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

controller/internal/web/templates/settings.html

@@ -166,6 +166,7 @@
 
     <div style="margin-top:1rem;display:flex;gap:.75rem;flex-wrap:wrap">
         <a href="/settings/storage/init" class="btn btn-sm btn-outline">🔧 Új meghajtó inicializálása</a>
+        <a href="/settings/storage/attach" class="btn btn-sm btn-outline">🔗 Meglévő meghajtó csatolása</a>
     </div>
 
     <details class="storage-add-details">