handleLoginReq: layer ACL between admin pw and guest/blank pw

Reorder login auth so that, in priority:

  1. Admin password match -> ADMIN (overrides ACL — a region_mgr or
     other ACL'd client who knows the admin pw can still upgrade).
  2. Sender pubkey is in the ACL -> use the stored permissions
     (no password required; pubkey is itself an identity assertion).
  3. Guest password match (or blank pw when guest pw is unset) -> GUEST.
  4. Otherwise -> reject.

Previously the ACL was consulted only when `data[0] == 0` (blank pw),
so a non-blank password would skip the ACL entirely and fall through
to the password block. Phones that sent any non-empty password field
— even a stale one — would never see their ACL-granted role; they'd
either be rejected or assigned the role implied by the password match.

Implementation is a single-line change: gate the ACL lookup on "the
admin pw did NOT match". When admin matches, we skip the lookup so
the existing password block sets ADMIN unconditionally (the right
behavior for the upgrade case). When admin does not match, the ACL
takes over; only on an ACL miss do we fall through to guest/reject.

examples/simple_repeater/MyMesh.cpp

@@ -89,7 +89,7 @@ void MyMesh::putNeighbour(const mesh::Identity &id, uint32_t timestamp, float sn
 
 uint8_t MyMesh::handleLoginReq(const mesh::Identity& sender, const uint8_t* secret, uint32_t sender_timestamp, const uint8_t* data, bool is_flood) {
   ClientInfo* client = NULL;
-  if (data[0] == 0) {   // blank password, just check if sender is in ACL
+  if (strcmp((char *)data, _prefs.password) != 0) {  // admin pw bypasses ACL (allows upgrade)
     client = acl.getClient(sender.pub_key, PUB_KEY_SIZE);
     if (client == NULL) {
     #if MESH_DEBUG