Fix fragment navigation for gistpreview.github.io URLs (#13)

> The gistpreview mechanis has a bug: fragment links like https://gistpreview.github.io/?edbd5ddcb39d1edc9e175f1bf7b9ef9a/page-001.html#msg-2025-12-26T15-30-45-910Z do not naught the user to the right point, presumably because the content has not loaded in time
>
> Fix that with more JavaScript in the existing gistpreview JavaScript

When accessing a gistpreview URL with a fragment (e.g. #msg-2025-12-26T15-30-45-910Z), the browser's native fragment navigation fails because gistpreview loads content dynamically. By the time the browser tries to scroll to the element, it doesn't exist yet.

Added JavaScript that:
- Checks for fragment in window.location.hash
- Uses scrollIntoView to navigate to the target element
- Retries with increasing delays (100ms, 300ms, 500ms, 1s) to handle dynamic content loading

https://gistpreview.github.io/?883ee001b0d63a2045f1e2c7c2598a9f/index.html