feat(webserver): confirm download/delete with file count in default template - #214

…emplate

Show the number of selected files in the confirmation dialogs and avoid
acting when nothing is selected, in the default web template.

- Search: extend formCommandSubmit's "download" path to count selected
  checkboxes (excluding the select-all master), return early when none
  are selected, and confirm "Download selected N files ?".
- Search: add "return false" to the Download submit button onClick so the
  native form submission no longer fires; formCommandSubmit submits the
  form itself. This fixes the page reloading even when the user pressed
  Cancel in the confirmation dialog.
- Downloads: replace the generic "Delete selected files ?" prompt with a
  count-aware "Delete selected N files ?", returning early when no rows
  are selected.