Automatically select added files (#4325)

# Description of Changes
When adding files, the user probably wants to use them straight away in
the next tool that they use, so automatically select any added files (in
addition to whatever they previously had selected).

frontend/src/contexts/file/fileActions.ts

@@ -88,6 +88,12 @@ interface AddFileOptions {
   insertAfterPageId?: string;
 }
 
+export interface AddedFile {
+  file: File;
+  id: FileId;
+  thumbnail?: string;
+}
+
 /**
  * Unified file addition helper - replaces addFiles/addProcessedFiles/addStoredFiles
  */
@@ -98,13 +104,13 @@ export async function addFiles(
   filesRef: React.MutableRefObject<Map<FileId, File>>,
   dispatch: React.Dispatch<FileContextAction>,
   lifecycleManager: FileLifecycleManager
-): Promise<Array<{ file: File; id: FileId; thumbnail?: string }>> {
+): Promise<AddedFile[]> {
   // Acquire mutex to prevent race conditions
   await addFilesMutex.lock();
 
   try {
   const fileRecords: FileRecord[] = [];
-  const addedFiles: Array<{ file: File; id: FileId; thumbnail?: string }> = [];
+  const addedFiles: AddedFile[] = [];
 
   // Build quickKey lookup from existing files for deduplication
   const existingQuickKeys = buildQuickKeySet(stateRef.current.files.byId);