fix: avoid infinite loop when hitting an inexistent route

[giacomocerquone]

closes #48

Testing the pr

When testing the pr, remember to

1. Add ElmBook.ThemeOptions.useHashBasedNavigation in the UI/ElmBook.elm file
2. Modify a chapter so to add an inexistent link to test the bugfix

FYI: The links must be in this format #/guides/inexistent

Rationale

When hashBasedNavigation is true and a route that can't be found among the chapters is hit, this is what happens:

1. OnUrlChange is triggered
2. extractPath url_ is called and returns /guides/inexistent (url.fragment) which is then assigned to url
3. The catch all "_" case executes
4. An urlChapter can't be found so it goes in the Nothing case
5. The Nothing case navigates to #/
6. The OnUrlChange is triggered again
7. extractPath url_ is called and returns / (url.fragment is Nothing hence Maybe.withDefault kicks in) which is then assigned to url
8. The "/" case executes
9. It gets the fallback chapter's url (/overview in the case of this repo) and navigates there, which is a problem because we have hashBasedNavigation set to true
10. The OnUrlChange is triggered again
11. extractPath url_ is called and when hashBasedNavigation is true it returns the fragment or / and in the case of /overview the fragment is Nothing.
12. From now on it repeats from 8 to 12 infinitely

Solution

How do we stop it? Should be simple honestly, when the fallback chapter is retrieved, let's add the hashtag on front.

Screencast

https://github.com/user-attachments/assets/669b83f2-15fd-409f-9774-57412e7940c1