feat: fetch the next page of books before the reader hits the bottom

The prefetch zone is a viewport deep rather than 200px, and the observer
re-observes after a page lands so a chain of loads is not cut short.

frontend/src/lib/state/bookCollection.svelte.ts

@@ -28,6 +28,8 @@ export class BookCollectionState {
 	public moreBooks = $state(false);
 	private currentBookPage = $state(1);
 	public loading = $state(false);
+	/** A page append, as opposed to `loading`, which replaces the whole list. */
+	public loadingMore = $state(false);
 
 	/**
 	 * Whether the reader has loaded past the first page. Anything that refetches
@@ -165,22 +167,37 @@ export class BookCollectionState {
 		});
 
 		this.books = [...result.items];
+		this.moreBooks = result.total > result.items.length;
 		this.currentBookPage = 1;
 		this.loading = false;
 	}
 
+	/**
+	 * Prefetching means the trigger can fire again while a page is still in
+	 * flight, so the guard is here rather than in the observer — every caller
+	 * gets it, and a second call is dropped instead of duplicating a page.
+	 */
 	async loadMoreBooks() {
-		const result = await this.ops.listBooks({
-			currentPage: this.currentBookPage + 1,
-			pageSize: 50,
-			sortOrder: this.sortOrder,
-			orderBy: this.orderBy,
-			...this.filters
-		});
+		if (this.loadingMore || !this.moreBooks) return;
 
-		this.books = [...this.books, ...result.items];
-		this.moreBooks = result.total > this.books.length;
-		this.currentBookPage++;
+		this.loadingMore = true;
+		try {
+			const result = await this.ops.listBooks({
+				currentPage: this.currentBookPage + 1,
+				pageSize: 50,
+				sortOrder: this.sortOrder,
+				orderBy: this.orderBy,
+				...this.filters
+			});
+
+			this.books = [...this.books, ...result.items];
+			// An empty page means the count we were given was stale; stop asking
+			// rather than loop on a page that never grows the list.
+			this.moreBooks = result.items.length > 0 && result.total > this.books.length;
+			this.currentBookPage++;
+		} finally {
+			this.loadingMore = false;
+		}
 	}
 
 	updateBooks(books: PaginatedResponse<Book>) {