[Feature Request] Bulk download: file name template, one merged PDF, table of contents

Description Three additions to the bulk download in the document list: 1. File names from a template — when downloading several documents as separate files (ZIP), let me set the file name with placeholders ({{ created }} {{ correspondent }} {{ title }} etc.), optionally numbered (001_, 002_ …) and sorted. 2. Download as one PDF — merge the selected documents into a single PDF for download, one bookmark per document. Not stored in the archive (unlike *Merge*, which creates a new document). 3. Table of contents — optional for both 1 and 2: a TOC page listing every document (title, correspondent, date, ASN). In the merged PDF each entry jumps to the page, in the ZIP it links to the file. Use case: handing a set of documents to someone else (tax advisor, insurance, lawyer) as one clean package instead of a ZIP of loose, unsorted files. Most building blocks already exist (bulk download, merge, filename placeholders from #12095), so I think this is not a big change. Related: #1977 (sin…