feat: merge books from the duplicates screen and the toolbar

A workbench with the folded records in a rail, the survivor's fields live and
editable beside them, and per-field actions chosen by what kind of field it is.
Fields the records agree on stay out of the way. Reachable from a duplicate
group and from the selection toolbar, which is the only way to merge a pair the
detector never proposed.

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

@@ -9,6 +9,7 @@ import {
 	editBookMetadataSchema,
 	updateBookProgressSchema,
 	duplicateDismissalSchema,
+	bookMergeSchema,
 	type Book,
 	type BooksUploadResult,
 	type DuplicateBookGroup,
@@ -174,6 +175,24 @@ export const listDuplicateBooks = query(
 	}
 );
 
+/**
+ * Fold several books into one and delete the records folded in.
+ *
+ * Irreversible, so the caller is expected to have shown what is about to happen.
+ */
+export const mergeBooks = command(
+	bookMergeSchema,
+	async ({ library_id, ...data }): Promise<Book> => {
+		const { locals } = getRequestEvent();
+
+		const response = await locals.api.post(`/books/merge?library_id=${library_id}`, data);
+
+		if (!response.ok) error(response.status, detailOf(await response.text()));
+
+		return await response.json();
+	}
+);
+
 /** Record that two books are not the same book, so the pair stops being proposed. */
 export const dismissDuplicateBooks = command(duplicateDismissalSchema, async (data) => {
 	const { locals } = getRequestEvent();