Fix button_to syntax for Rails 8 compatibility

Fixed "undefined method stringify_keys for String" error caused by
incorrect button_to syntax when using block form.

Changes:
- Remove text argument from button_to when using block
- Block content becomes button text in Rails 8 syntax
- Correct syntax: button_to url, options do ... end
- Incorrect syntax: button_to "text", url, options do ... end

This fixes the NoMethodError on the login page.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

app/views/layouts/shared/_header.html.erb

@@ -81,7 +81,7 @@
 
         <div class="col-auto">
           <div class="d-flex align-items-center gap-2">
-            <%= button_to "Tutorial Credentials", "#", {
+            <%= button_to "#", {
               id: "show_creds_btn",
               class: "btn btn-sm btn-warning",
               method: "get",