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/schema/book.ts

@@ -6,6 +6,15 @@ export type Book = components['schemas']['BookRead'];
 export type BookFile = components['schemas']['FileMetadataRead'];
 export type BookProgress = components['schemas']['BookProgressRead'];
 
+/**
+ * A file the library already held, so the upload did not store it again.
+ *
+ * `book_id` is null when the match was another file in the same upload — there is
+ * no book to point at yet.
+ */
+export type DuplicateFile = components['schemas']['DuplicateFileRead'];
+export type BooksUploadResult = components['schemas']['BooksUploadResult'];
+
 export const bookQuerySchema = commonQuerySchema.extend({
 	libraries: stringArrayCoerce,
 	authors: stringArrayCoerce,