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/components/forms/edit-book/edit-files.svelte

@@ -50,6 +50,22 @@
 		toast.error(`${file.name} was not added`, { description: reason });
 	};
 
+	/**
+	 * The API's own words, when it has any.
+	 *
+	 * Adding a file the library already holds under another book is refused with a
+	 * 409 naming it — far more use than "failed to add files". SvelteKit hands an
+	 * `error()` back as an HttpError on the client, so the message sits on `body`.
+	 */
+	function apiMessage(error: unknown): string | undefined {
+		if (typeof error !== 'object' || error === null) return undefined;
+
+		const body = (error as { body?: { message?: string } }).body;
+		if (typeof body?.message === 'string') return body.message;
+
+		return error instanceof Error ? error.message : undefined;
+	}
+
 	function confirmDelete(file: BookFile) {
 		fileToDelete = file;
 		confirmOpen = true;
@@ -148,7 +164,8 @@
 				toast.success('Files added');
 			} catch (error) {
 				console.error('Failed to add files: ', error);
-				toast.error('Failed to add files');
+				toast.error(apiMessage(error) ?? 'Failed to add files');
+				uploadBookFiles.fields.files.set([]);
 			}
 		})}
 		enctype="multipart/form-data"