fix multi-byte string processing

src/Entity/App.php

@@ -158,7 +158,7 @@ class App
         if ($value)
         {
             $title = urldecode(
-                strlen($value) > $this->config->header->title->length->max ? substr($value, 0, $this->config->header->title->length->max) . '...'
+                mb_strlen($value) > $this->config->header->title->length->max ? mb_substr($value, 0, $this->config->header->title->length->max) . '...'
                                                                               : $value
             );
         }

src/Entity/Tab/Page.php

@@ -640,7 +640,7 @@ class Page
         if ($value)
         {
             $title = urldecode(
-                strlen($value) > $this->config->title->length->max ? substr($value, 0, $this->config->title->length->max) . '...'
+                mb_strlen($value) > $this->config->title->length->max ? mb_substr($value, 0, $this->config->title->length->max) . '...'
                                                                       : $value
             );
         }