Fix remaining button_to syntax errors for Rails 8

Fixed "stringify_keys for String" errors on Sign Up and Login buttons
by removing text arguments from button_to when using block form.

Changes:
- Fix Sign Up button: button_to signup_path (not "Sign Up", signup_path)
- Fix Login button: button_to login_path (not "Login", login_path)
- Block content now provides button text in Rails 8

All button_to calls now use correct Rails 8 syntax.

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

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

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

@@ -98,14 +98,14 @@
               <i class="bi bi-book"></i> Tutorials
             <% end %>
 
-            <%= button_to "Sign Up", signup_path, {
+            <%= button_to signup_path, {
               class: "btn btn-sm btn-primary",
               method: "get"
             } do %>
               <i class="bi bi-person-plus"></i> Sign Up
             <% end %>
 
-            <%= button_to "Login", login_path, {
+            <%= button_to login_path, {
               class: "btn btn-sm btn-outline-primary",
               method: "get"
             } do %>