Add last modified tracking: show 'Last modified: date by username' in document viewer

- Added modified_by column to files table
- Updated WOPI PutFile to track who modified the file
- Updated view handlers to return file metadata (name, size, lastModified, modifiedByName)
- Updated Flutter models and UI to display last modified info

go_cloud/internal/http/wopi_handlers.go

@@ -512,7 +512,7 @@ func wopiPutFileHandler(w http.ResponseWriter, r *http.Request, db *database.DB,
 
 	// Update file size and modification time in database
 	newSize := int64(len(content))
-	err = db.UpdateFileSize(r.Context(), fileUUID, newSize)
+	err = db.UpdateFileSize(r.Context(), fileUUID, newSize, &userID)
 	if err != nil {
 		fmt.Printf("[WOPI-STORAGE] Failed to update file size: file=%s error=%v\n", fileID, err)
 		// Don't fail the upload, just log the warning