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/css/styles.css

@@ -625,6 +625,21 @@ header {
   font-size: 0.875rem;
 }
 
+/* Testing Controls */
+.testing-controls {
+  opacity: 0.5;
+  font-size: 0.8rem;
+}
+
+.testing-controls:hover {
+  opacity: 1;
+}
+
+.testing-controls a {
+  text-decoration: underline;
+  color: var(--text-color);
+}
+
 /* Utilities */
 .text-center { text-align: center; }
 .text-right { text-align: right; }