Replace modal with dedicated credentials page

Removed complex modal implementation and replaced with simple
link to dedicated credentials page to eliminate all modal issues.

Changes:
- Add credentials action to TutorialsController
- Remove layout false restriction for credentials
- Replace button with simple link_to for Demo Credentials
- Remove entire modal HTML structure
- Remove all JavaScript for modal initialization
- Remove fetch/AJAX complexity

The credentials view already existed but was modal-only. Now it's
a proper page that users can navigate to directly. Much simpler!

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

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

app/controllers/tutorials_controller.rb

@@ -3,7 +3,9 @@ class TutorialsController < ApplicationController
   skip_before_action :has_info
   skip_before_action :authenticated
 
-  layout false, only: [:credentials]
+  def credentials
+    # Render credentials page with layout
+  end
 
   # VULNERABILITY: Regular Expression Denial of Service (ReDoS)
   # This endpoint demonstrates how malicious input can cause catastrophic backtracking