fix: clean up stale raw mounts before scanning in attach wizard

After an interrupted attach wizard, the raw mount stays behind,
causing the device to appear as "mounted" in scan results. Now the
scan button calls cancel first, which unmounts any stale raw mounts
that have no bind mount in fstab.

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

controller/internal/web/templates/storage_attach.html

@@ -167,7 +167,11 @@ function scanDisks() {
     errEl.style.display = 'none';
     resultEl.style.display = 'none';
 
-    fetch('/api/storage/scan', {method:'POST'})
+    // Clean up any stale raw mounts from interrupted previous sessions first,
+    // so the device appears as available in the scan results.
+    fetch('/api/storage/attach/cancel', {method:'POST'})
+        .catch(function(){}) // ignore cancel errors
+        .then(function() { return fetch('/api/storage/scan', {method:'POST'}); })
         .then(function(r){ return r.json(); })
         .then(function(data) {
             btn.textContent = '🔍 Meghajtók keresése';