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/components/view/book-browser.svelte

@@ -29,19 +29,31 @@
 
 	$effect(() => {
 		if (!sentinel || !bookCollection.moreBooks) return;
+		const target = sentinel;
+
 		const observer = new IntersectionObserver(
 			async (entries) => {
-				if (entries[0].isIntersecting && !bookCollection.loading) {
+				if (!entries[0].isIntersecting) return;
+
 				await bookCollection.loadMoreBooks();
+
+				// A page that does not push the sentinel back out of the prefetch zone
+				// produces no further intersection change, so the observer would sit
+				// idle until the next scroll. Re-observing asks for a fresh reading.
+				if (bookCollection.moreBooks) {
+					observer.unobserve(target);
+					observer.observe(target);
 				}
 			},
 			{
 				root: scrollContainer,
-				rootMargin: '0px 0px 200px 0px' 
+				// One viewport of lead time: the next page is requested a full screen
+				// before the reader reaches the end of the current one.
+				rootMargin: '0px 0px 100% 0px'
 			}
 		);
 
-		observer.observe(sentinel);
+		observer.observe(target);
 
 		return () => {
 			observer.disconnect();
@@ -110,7 +122,7 @@
 
 						{#if bookCollection.moreBooks}
 							<div bind:this={sentinel}>
-								{#if bookCollection.loading}
+								{#if bookCollection.loadingMore}
 									<Spinner />
 								{/if}
 							</div>

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
@@ -165,11 +167,21 @@ 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() {
+		if (this.loadingMore || !this.moreBooks) return;
+
+		this.loadingMore = true;
+		try {
 			const result = await this.ops.listBooks({
 				currentPage: this.currentBookPage + 1,
 				pageSize: 50,
@@ -179,8 +191,13 @@ export class BookCollectionState {
 			});
 
 			this.books = [...this.books, ...result.items];
-		this.moreBooks = result.total > this.books.length;
+			// 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>) {