fix(ui): stop the book page refetching forever for a book that does not exist

[jordanfelle]

Problem

Opening /book/<id> for a book that no longer exists (deleted, or pruned by a metadata profile) shows a spinner indefinitely instead of the existing "Sorry, that book cannot be found" screen.

Cause

BookDetailsPageConnector fetches the book when it is not in the store (needsBookFetch = !isFetching && !hasBook). For a missing book the server answers 404 (GET /book?bookId=<id>; checked against a live instance: 404 in ~1 ms). The FETCH_BOOKS failure branch sets isFetching: false, so needsBookFetch flips false -> true, and componentDidUpdate treats that transition as a reason to populate() again. The result is an endless loop of failing requests with the spinner up most of the time, so the NotFound render at the bottom of render() is never reached.

Change

• A definitive 404 is final: the book is fetched once and the existing NotFound screen renders.
• Anything else (a transient 500, a network error, a request aborted by another fetchBooks, an empty 200) is retried, up to 3 fetches per route (MAX_BOOK_FETCH_ATTEMPTS), so a persistent failure still ends on NotFound instead of looping, and a transient one still recovers.
• The counter resets when the route key changes. The connector now also reads books.error to tell a 404 from other failures.
• The author fetch gets a once-per-author guard, since it has the same shape if an author is missing.

Testing

• Syntax-checked with esbuild; the full webpack production build compiles.
• I modelled the render/componentDidUpdate cycle in plain JavaScript (this checks the reasoning, not the React component): book deleted (404 every time) = 1 fetch then NotFound; transient 500 then success = 2 fetches then the book; aborted request then success = 2 fetches then the book; persistent 500 = 3 fetches then NotFound; empty 200 forever = 3 fetches then NotFound; book exists = 1 fetch. Without the guard the 404 case fetches on every cycle (stopped by my cap at 40).
• No frontend tests exist in this repo, and I have not yet checked it in a browser.