feat: report skipped duplicates in the upload tray

A book whose files were all already stored settles as skipped rather than done,
naming the book that holds them and offering to add it anyway.

frontend/src/lib/api/book.remote.ts

@@ -9,11 +9,27 @@ import {
 	editBookMetadataSchema,
 	updateBookProgressSchema,
 	type Book,
+	type BooksUploadResult,
 	bookFilesUpload
 } from '$lib/schema/index';
 import { stringCoerce, type PaginatedResponse } from '$lib/schema/common';
 import { createQueryParams } from '$lib/utils';
 
+/**
+ * The backend's own message for a failed response, rather than its JSON envelope.
+ *
+ * A refused duplicate answers 409 with a `detail` worth reading and the offending
+ * files in `extra`; passing the body through whole puts JSON in front of the reader.
+ */
+function detailOf(body: string): string {
+	try {
+		const parsed = JSON.parse(body);
+		return typeof parsed?.detail === 'string' ? parsed.detail : body;
+	} catch {
+		return body;
+	}
+}
+
 export const getBook = query(stringCoerce, async (id): Promise<Book> => {
 	const { locals } = getRequestEvent();
 
@@ -68,26 +84,29 @@ export const updateBookCover = form(bookCoverUpload, async ({ book_id, file }) =
 	return await response.json();
 });
 
-export const uploadBooks = form(booksUpload, async ({ library_id, files }) => {
-	const { locals } = getRequestEvent();
+export const uploadBooks = form(
+	booksUpload,
+	async ({ library_id, files }): Promise<BooksUploadResult> => {
+		const { locals } = getRequestEvent();
 
-	const formData = new FormData();
-	files.forEach((file) => {
-		formData.append('files', file);
-	});
+		const formData = new FormData();
+		files.forEach((file) => {
+			formData.append('files', file);
+		});
 
-	const response = await locals.api.postMultipart(
-		`/books/fromFiles?library_id=${library_id}`,
-		formData
-	);
+		const response = await locals.api.postMultipart(
+			`/books/fromFiles?library_id=${library_id}`,
+			formData
+		);
 
-	if (!response.ok) {
-		const message = await response.text();
-		error(response.status, message);
+		if (!response.ok) {
+			const message = await response.text();
+			error(response.status, message);
+		}
+
+		return await response.json();
 	}
-
-	return await response.json();
-});
+);
 
 export const uploadBookFiles = form(bookFilesUpload, async ({ book_id, files }) => {
 	const { locals } = getRequestEvent();
@@ -100,8 +119,9 @@ export const uploadBookFiles = form(bookFilesUpload, async ({ book_id, files })
 	const response = await locals.api.postMultipart(`/books/${book_id}/files`, formData);
 
 	if (!response.ok) {
-		const message = await response.text();
-		error(response.status, message);
+		// 409 here means the file is already stored under a different book, which is
+		// something the reader can act on — so the message has to survive the trip.
+		error(response.status, detailOf(await response.text()));
 	}
 
 	return await response.json();