feat: optional 4-digit PIN asked in every new tab (not on reload), on-screen keypad for phones; set, change or remove it in the account; five misses sign out

internal/server/web/templates/settings.html

@@ -5,6 +5,7 @@
 <meta name="viewport" content="width=device-width, initial-scale=1">
 <title>Настройки - LPRA</title>
 <link rel="stylesheet" href="{{asset "app.css"}}">
+<script src="{{asset "pin.js"}}"></script>
 <style>
   /* An install command, with a button to copy it at its right. */
   .copybox { position:relative; }