File Manager: Finalize migration to JSON (file_manager API)

[mgutt]

With PR #2429 we changed some API responses of file_manager to JSON as for example:

        $reply['status'] = json_encode([
          'action' => $action,
          'text' => [htmlspecialchars(mb_strimhalf(exec("tail -1 $status"), 70, '...'), ENT_QUOTES, 'UTF-8')]
        ]);

And we added the processing part to BrowseButton.page as follows:

function dfm_showProgress(data) {
  if (!data) return 0;

  // Try to parse as JSON first
  try {
    let parsed = JSON.parse(data);

    // Universal JSON format: {action: int, text: [text0, text1]}
    // text[0] = file/main text, text[1] = progress info (optional)
    if (parsed.action !== undefined && Array.isArray(parsed.text)) {
...

We should complete this change and return JSON only. Steps:

• obtain all actions still returning raw HTML in file_manager
• change their response to JSON
• check if dfm_showProgress() needs additional code to handle the response (e.g. add switch case per action id)
• remove legacy code from dfm_showProgress() which handles raw HTML