fix: restore Firefox drag-and-drop support and fix webkitRelativePath handling

- Add fallback to event.dataTransfer.files when webkitGetAsEntry() returns null
  (fixes Firefox drag-and-drop regression)
- Set webkitRelativePath on files obtained via FileSystemFileEntry for consistency
- Strip leading slash from entry.fullPath in both addFromDir and handleDroppedItems
- Add folder selection button with webkitdirectory input for cross-browser folder uploads

Author: ErykKul

src/sections/shared/file-uploader/file-upload-input/FileUploadInput.tsx

@@ -143,14 +143,15 @@ const FileUploadInput = ({ fileRepository, datasetPersistentId }: FileUploadInpu
     }
 
     const droppedItems = event.dataTransfer.items
+    const droppedFiles = event.dataTransfer.files
 
     if (droppedItems.length > 0) {
       if (operationType === OperationType.REPLACE_FILE && droppedItems.length > 1) {
         toast.error(t('fileUploader.replaceFileMultipleError'))
         return
       }
 
-      handleDroppedItems(droppedItems)
+      handleDroppedItems(droppedItems, droppedFiles)
     }
   }
 

src/sections/shared/file-uploader/useFileUploadOperations.ts

@@ -174,7 +174,7 @@ export function useFileUploadOperations(config: FileUploadOperationsConfig): Fil
               })
 
               Object.defineProperty(fileWithPath, 'webkitRelativePath', {
-                value: entry.fullPath,
+                value: entry.fullPath.startsWith('/') ? entry.fullPath.slice(1) : entry.fullPath,
                 writable: true
               })
 
@@ -202,7 +202,16 @@ export function useFileUploadOperations(config: FileUploadOperationsConfig): Fil
           handledViaEntry = true
           const fse = entry as FileSystemFileEntry
           fse.file((file) => {
-            void uploadOneFile(file)
+            // Create a new File with webkitRelativePath set for consistency
+            const fileWithPath = new File([file], file.name, {
+              type: file.type,
+              lastModified: file.lastModified
+            })
+            Object.defineProperty(fileWithPath, 'webkitRelativePath', {
+              value: entry.fullPath.startsWith('/') ? entry.fullPath.slice(1) : entry.fullPath,
+              writable: true
+            })
+            void uploadOneFile(fileWithPath)
           })
         }
       })