feat: add PDF overlay and underlay functionality

src/js/types/index.ts

@@ -65,3 +65,4 @@ export * from './shortcuts-type.ts';
 export * from './ui-type.ts';
 export * from './markdown-editor-type.ts';
 export * from './sanitize-type.ts';
+export * from './overlay-pdf-type.ts';

src/js/types/overlay-pdf-type.ts

@@ -0,0 +1,4 @@
+export interface OverlayPdfState {
+  baseFile: File | null;
+  overlayFile: File | null;
+}