Add Merge UI to V2 (#4235)

# Description of Changes
Add UI for Merge into V2.

frontend/src/contexts/file/FileReducer.ts

@@ -145,12 +145,17 @@ export function fileContextReducer(state: FileContextState, action: FileContextA
 
       // Validate that all IDs exist in current state
       const validIds = orderedFileIds.filter(id => state.files.byId[id]);
-
+      // Reorder selected files by passed order
+      const selectedFileIds = orderedFileIds.filter(id => state.ui.selectedFileIds.includes(id));
       return {
         ...state,
         files: {
           ...state.files,
           ids: validIds
+        },
+        ui: {
+          ...state.ui,
+          selectedFileIds,
         }
       };
     }
@@ -234,11 +239,14 @@ export function fileContextReducer(state: FileContextState, action: FileContextA
 
     case 'CONSUME_FILES': {
       const { inputFileIds, outputStirlingFileStubs } = action.payload;
+
       return processFileSwap(state, inputFileIds, outputStirlingFileStubs);
     }
 
+
     case 'UNDO_CONSUME_FILES': {
       const { inputStirlingFileStubs, outputFileIds } = action.payload;
+
       return processFileSwap(state, outputFileIds, inputStirlingFileStubs);
     }
 

frontend/src/contexts/file/fileHooks.ts

@@ -136,13 +136,13 @@ export function useAllFiles(): { files: StirlingFile[]; records: StirlingFileStu
 /**
  * Hook for selected files (optimized for selection-based UI)
  */
-export function useSelectedFiles(): { files: StirlingFile[]; records: StirlingFileStub[]; fileIds: FileId[] } {
+export function useSelectedFiles(): { selectedFiles: StirlingFile[]; selectedRecords: StirlingFileStub[]; selectedFileIds: FileId[] } {
   const { state, selectors } = useFileState();
 
   return useMemo(() => ({
-    files: selectors.getSelectedFiles(),
-    records: selectors.getSelectedStirlingFileStubs(),
-    fileIds: state.ui.selectedFileIds
+    selectedFiles: selectors.getSelectedFiles(),
+    selectedRecords: selectors.getSelectedStirlingFileStubs(),
+    selectedFileIds: state.ui.selectedFileIds
   }), [state.ui.selectedFileIds, selectors]);
 }
 
@@ -169,7 +169,6 @@ export function useFileContext() {
     recordOperation: (_fileId: FileId, _operation: any) => {}, // Operation tracking not implemented
     markOperationApplied: (_fileId: FileId, _operationId: string) => {}, // Operation tracking not implemented
     markOperationFailed: (_fileId: FileId, _operationId: string, _error: string) => {}, // Operation tracking not implemented
-
     // File ID lookup
     findFileId: (file: File) => {
       return state.files.ids.find(id => {