Add update testing functionality

- Add test mode for simulating available updates
- Implement a toggle to enable/disable test mode
- Add test parameter to check-updates endpoint
- Clean up debugging comments
- Create subtle testing UI controls

This enables testing the update functionality without needing
to create a new higher version in the repository.

🤖 Generated with [Claude Code](https://claude.ai/code)
Co-Authored-By: Claude <noreply@anthropic.com>

public/index.html

@@ -133,6 +133,10 @@
                                                     <i class="fas fa-circle-notch fa-spin"></i> Checking...
                                                 </span>
                                             </div>
+                                            <!-- Testing Controls (hidden in production) -->
+                                            <div class="mt-2 testing-controls">
+                                                <small><a href="#" id="toggle-test-update-button">Toggle Test Update</a></small>
+                                            </div>
                                             <div id="update-available" class="mt-3 d-none">
                                                 <div class="alert alert-info">
                                                     <i class="fas fa-arrow-circle-up"></i>