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/schema/book.ts

@@ -30,6 +30,19 @@ export type PossibleDuplicate = components['schemas']['PossibleDuplicateRead'];
 /** Books the library holds that all look like copies of one book. */
 export type DuplicateBookGroup = components['schemas']['DuplicateBookGroupRead'];
 
+/** The metadata a reader resolved while merging, or while reviewing a provider. */
+export type BookMetadataUpdate = components['schemas']['BookMetadataUpdate'];
+
+/** Mirrors BookMerge in backend/src/chitai/schemas/book.py */
+export const bookMergeSchema = z.object({
+	library_id: z.coerce.number(),
+	survivor_id: z.coerce.number(),
+	merged_ids: z.array(z.coerce.number()).min(1, 'Pick at least one book to fold in'),
+	// Passed through untouched — the backend validates it as BookMetadataUpdate, and
+	// duplicating that shape here would be two places to keep in step for no gain.
+	metadata: z.record(z.string(), z.unknown()).optional()
+});
+
 /** Mirrors DuplicateDismissal in backend/src/chitai/schemas/book.py */
 export const duplicateDismissalSchema = z.object({
 	book_a_id: z.coerce.number(),
@@ -128,4 +141,5 @@ export type BookEditMetadata = z.infer<typeof editBookMetadataSchema>;
 export type DeleteBook = z.infer<typeof deleteBooksSchema>;
 export type DeleteBookFiles = z.infer<typeof deleteBookFilesSchema>;
 export type DuplicateDismissal = z.infer<typeof duplicateDismissalSchema>;
+export type BookMerge = z.infer<typeof bookMergeSchema>;
 export type UpdateBookProgress = z.infer<typeof updateBookProgressSchema>;