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/server.go

@@ -40,6 +40,9 @@ type Server struct {
 	// Disk operation state (format/migrate jobs)
 	diskJobMu sync.Mutex
 	diskJob   *activeDiskJob
+
+	// Active raw mount for the attach wizard (empty when not in use)
+	activeRawMount string
 }
 
 func NewServer(cfg *config.Config, stackMgr *stacks.Manager, cpuCollector *system.CPUCollector, backupMgr *backup.Manager, crossDrive *backup.CrossDriveRunner, sched *scheduler.Scheduler, sett *settings.Settings, alertMgr *AlertManager, notif *notify.Notifier, logger *log.Logger, version string) *Server {
@@ -117,6 +120,8 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		s.backupRestoreHandler(w, r)
 	case path == "/settings/storage/init":
 		s.storageInitHandler(w, r)
+	case path == "/settings/storage/attach":
+		s.storageAttachHandler(w, r)
 	case strings.HasPrefix(path, "/stacks/") && strings.HasSuffix(path, "/migrate"):
 		name := strings.TrimPrefix(path, "/stacks/")
 		name = strings.TrimSuffix(name, "/migrate")