Broken external preview linking

### Problem

Dear developers,

Thank you for looking into this issue.

### Reproducing the bug

1. Attach a weblink to the book, with the link type being specified as "Preview":

Since https://gallica.bnf.fr/ark:/12148/bpt6k3398372s only allows reading 15% of the entire book, the "Preview" link type seems appropriate.

2. Tap the "Preview" button on the book page:

3. You are sent to an error page instead of the link you attached in step 1.

* Expected behavior: Being transferred to the weblink you attached to the book.
* Actual behavior: Being transferred to an error page.

### Context

- Browser (Chrome, Safari, Firefox, etc): Chrome
- OS (Windows, Mac, etc): Android
- Logged in (Y/N): Y
- Environment (prod, dev, local): prod

### Breakdown

Implementation Details (for maintainers)

#### Requirements Checklist
* [ ]

#### Related files

*

#### Stakeholders

*

#### Instructions for Contributors

- Please [run these commands](https://docs.openlibrary.org/developers/tools/git.html#working-on-your-branch) to ensure your repository is up to date **before** [creating a new branch](https://docs.openlibrary.org/developers/tools/git.html#making-changes-and-creating-a-pull-request) to work on this issue and **each time after** pushing code to Github, because the pre-commit bot may add commits to your PRs upstream.

Reproduce the issue using a book with a Gallica URL configured as a “Preview” weblink, then trace the preview button's link handling from the book page. Confirm the fix by verifying that tapping “Preview” opens the attached URL instead of an error page on Android Chrome.

Written by the indexing model from the issue text.