fix: preserve accumulated files in the Update Selected Questions modal

[Copilot]

The file upload widget in the "Update Selected Questions" modal was replacing the FileList on each picker open, making it impossible to build up a multi-file selection incrementally. The same DataTransfer-based accumulator pattern already used in new.html was absent from update_modal.html.

Changes

• update_modal.html
  • Adds a fileAccumulator (DataTransfer) scoped to the modal's JS closure.
  • fileUpload change handler now appends newly-picked files into the accumulator and writes the accumulated FileList back to the input — instead of replacing.
  • renderAccumulatedFileList() renders per-file badge+remove buttons below the input using textContent (XSS-safe).
  • removeAccumulatedFile(name) rebuilds the accumulator without the named file.
  • clearFileAccumulator() resets accumulator, input, and rendered list; called on modal hidden.bs.modal and Cancel click (replacing the previous bare fileUpload.value = '').
  • isFileSelected() now checks fileAccumulator.files.length instead of fileUpload.files.length.

// Before — each picker open replaced the previous selection
fileUpload.addEventListener('change', function() {
  if (updateModal.classList.contains('show')) {
    updateSaveButtonState();
  }
});

// After — files are accumulated across picker opens
fileUpload.addEventListener('change', function() {
  Array.from(this.files).forEach(f => fileAccumulator.items.add(f));
  fileUpload.files = fileAccumulator.files;
  renderAccumulatedFileList();
  if (updateModal.classList.contains('show')) updateSaveButtonState();
});