Overhaul the "PDF cover page" tab and fix two thumbnail-refresh bugs:

- Lazy previews: new pwg.pdfcp.renderPreview web-service rasterizes one page at
  a time on demand (admin only). The tab opens instantly and fills each preview
  slot over AJAX with a spinner instead of blocking on up to 10 ImageMagick
  renders. Adds a "Load 10 more pages" control for longer PDFs.

- Fix stale cover thumbnails: representative derivative URLs now carry a
  ?v=<mtime> cache-buster (pdfcp_bust_representative_url, priority 60 so it
  appends after the personal plugin's URL encoder).

- Fix covers not refreshing for filenames with [ or ]: core
  delete_element_derivatives() globs the path and glob() treats []/[] as a
  character class, deleting nothing. Replace it with a literal-prefix scan
  (pdfcp_purge_derivatives) in the new include/functions.inc.php helpers.

- Setting a cover redirects to Properties, carrying the confirmation as a
  page_inf...