Empty shelves on My Books page provide no guidance for new users

### Description:
When a user visits their "My Books" page with empty shelves, the UI displays only:

"No books are on this shelf"

There are no call-to-actions or links to help users understand how to add books.

### Steps to reproduce:
1. Login with a new or low-activity account
2. Visit /people/{username}/books
3. Observe empty sections like "My Loans" or "Currently Reading"

### Expected behavior:
Users should see guidance such as:
- Search for books
- Browse trending books
- Add their first book

### Actual behavior:
Only static text is shown with no actionable next steps.

### Suggested improvement:
Add contextual CTAs (e.g., link to /search or trending books) inside the empty state.

### File:
openlibrary/templates/account/mybooks.html

### Impact:
This is the first experience after login and can block user engagement.

### Breakdown

Implementation Details (for maintainers)

#### Related files

Refer to [this map of common Endpoints](https://docs.openlibrary.org/developers/backend/endpoints.html):
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#### Requirements Checklist
Checklist of requirements that need to be satisfied in order for this issue to be closed:
* [ ]

#### Stakeholders

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