feat: paginate chapter list and track reading progress in localStorage

Chapter list:
- Book page now shows the first 50 chapters only; a 'Load more' button
  (HTMX, GET /books/{slug}/chapters-page?page=N) appends the next page
  without a full navigation, replacing itself with the next load-more
  button or disappearing when all chapters are loaded.

Reading progress:
- Visiting a chapter saves {slug: chapterN} to localStorage under
  'reading_progress'.
- The book page reads that key on load and, if a saved chapter exists,
  highlights the saved chapter row with an amber dot and shows a
  'Resume — Chapter N' button that navigates directly to it.
- Progress dots are also re-applied after each Load More via
  hx-on::after-request.

scraper/internal/server/server.go

@@ -62,6 +62,7 @@ func (s *Server) ListenAndServe(ctx context.Context) error {
 	mux.HandleFunc("GET /ranking/view", s.handleRankingView)
 	mux.HandleFunc("GET /books/{slug}", s.handleBook)
 	mux.HandleFunc("GET /books/{slug}/chapters/{n}", s.handleChapter)
+	mux.HandleFunc("GET /books/{slug}/chapters-page", s.handleBookChaptersPage)
 	mux.HandleFunc("POST /ui/scrape/book", s.handleUIScrapeBook)
 	mux.HandleFunc("GET /ui/scrape/status", s.handleUIScrapeStatus)
 	// Plain-text chapter content for browser-side TTS