refactor: Phase D — composant Modal réutilisable + utilitaire searchParams

- Crée Modal.tsx dans components/ui (backdrop, container, header sticky, close button)
- Remplace le scaffolding modal dupliqué dans EditBookForm, EditSeriesForm,
  DeleteBookButton, MetadataSearchModal (4 composants)
- Crée lib/searchParams.ts avec paramString, paramStringOr, paramInt, paramBool
- Simplifie le parsing des query params dans books, series, authors pages

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/backoffice/app/components/ui/index.ts

@@ -21,3 +21,4 @@ export { PageIcon, NavIcon, Icon } from "./Icon";
 export { CursorPagination, OffsetPagination } from "./Pagination";
 export { Tooltip } from "./Tooltip";
 export { toast, Toaster } from "./Toast";
+export { Modal } from "./Modal";