Enable Vercel Speed Insights tracking

# Vercel Speed Insights Implementation

## Summary
Successfully implemented Vercel Speed Insights for the HTML-based project following the official Vercel documentation.

## Changes Made

### Modified Files
- **index.html** - Added Vercel Speed Insights script tags

## Implementation Details

According to the Vercel Speed Insights guide for HTML projects, I added the required script tags before the closing `</body>` tag:

1. **Initialization Script**: Added the `window.si` initialization function that queues Speed Insights calls
2. **Tracking Script**: Added the deferred loading of the Speed Insights script from `/_vercel/speed-insights/script.js`

## Additional Fixes

The original `index.html` file was missing closing `</body>` and `</html>` tags. These have been added to ensure proper HTML structure along with the Speed Insights implementation.

## Code Added

```html
<script>
    window.si = window.si || function () { (window.siq = window.siq || []).push(arguments); };
</script>
<script defer src="/_vercel/speed-insights/script.js"></script>

</body>
</html>
```

## Next Steps

Once deployed to Vercel with Speed Insights enabled:
1. The `/_vercel/speed-insights/*` routes will be automatically added
2. Performance metrics will start being collected
3. Data can be viewed in the Vercel dashboard under the Speed Insights tab

## Notes

- No package installation was required for the HTML implementation
- The scripts are loaded with the `defer` attribute to avoid blocking page rendering
- Speed Insights will only function when deployed to Vercel with the feature enabled in the project settings

Co-authored-by: Vercel <vercel[bot]@users.noreply.github.com>

Author: vercel[bot]

index.html

@@ -533,3 +533,11 @@ <h5 class="font-semibold mt-3">Passi suggeriti:</h5>
     });
 
 </script>
+
+<script>
+    window.si = window.si || function () { (window.siq = window.siq || []).push(arguments); };
+</script>
+<script defer src="/_vercel/speed-insights/script.js"></script>
+
+</body>
+</html>