Reader reference popup

I now have a prototype of the reference popup that allows users to quickly view the abstract of an in-text citation, add the item to their library, or open it if it already exists.

Reference popup with multiple references.

https://github.com/user-attachments/assets/d93b4bb1-5374-4378-9233-a4d1f3587810

1. Hover over an in-text citation to open a popup.
2. A list of raw reference texts is shown, with only two lines visible (inspired by the annotation sidebar’s text/comment collapsing behavior).
3. Click a reference to trigger recognition.
4. A loading indicator line appears.
5. Once recognized, the item is displayed, and the raw reference text turns gray to indicate it is no longer the primary focus.
6. Clicking on the raw reference text expands it (again, the same as in the sidebar annotation view).
7. Scrolled to another reference. To keep things clear, we might limit abstract visibility to 4 lines, so scrolling is more convenient.
8. Click another reference to recognize and load the item.
9. Click “Add to Library” to begin adding the item (along with its attachment) to the library. A circular loading indicator appears.
10. Once the item is added, it can be opened immediately in the reader.

Reference popup with a single reference. The key difference is that recognition starts automatically as soon as the popup is displayed.

https://github.com/user-attachments/assets/7c6ffb03-2ea3-410e-9136-3d959a208215

If the reference can’t be recognized or matched with an existing item, a Google Scholar search button appears. The raw reference text is expanded and stays black so the user can read it, copy it, or click any links.

https://github.com/user-attachments/assets/e9ddb634-519a-43fd-b9de-a6b90d47eee7

@dstillman @AbeJellinek @yexingsha and others, let me know if this looks good. If so, we can move on to finalizing the popup design.