Add hidden setting to puzzles

#19

app/views/puzzles/_form.html.slim

@@ -21,6 +21,13 @@
       .form-floating
         = form.number_field :pieces, autocomplete: "off", class: "form-control"
         = form.label :pieces, class: "required"
+  .row.mb-3
+    .col
+      .form-check.form-switch
+        = form.check_box :hidden, class: "form-check-input"
+        = form.label :hidden
+      .form-text
+        = t("activerecord.attributes.puzzle.hidden_description")
   .row.mb-3
     .col
       .form-text.mb-1
@@ -42,7 +49,7 @@
         }
 
         setMaxUploadSize();
-  .row.mt-4
+  .row.mt-4.mb-5
     .col
       - if method == :patch
         = link_to t("helpers.buttons.delete"), contest_puzzle_path(contest, puzzle), data: { turbo_method: :delete }, class: "btn btn-danger me-2"