[Feature Request] Choose which page is used for a document's thumbnail

Description Today the thumbnail/preview for every document is generated from page 1. For many scans the meaningful page isn't the first one (cover sheets, fax headers, blank separator pages, envelopes scanned first, multi-receipt batches), so the document list and cards show an unhelpful image. Proposal: a per-document "thumbnail page" setting. • API: an optional integer field on the document (e.g. thumbnail_page, default null = page 1), editable like other document fields. • UI: a page selector in document details (e.g. "Use this page as thumbnail" in the preview pane). • Saving the setting regenerates the thumbnail from that page. • The choice is kept across reprocessing, thumbnail rebuilds (document_thumbnails), and export/import; if the page no longer exists it falls back to page 1. This was previously requested in #12035 ("Allow select a page as important"), which was auto-closed for lack of votes and is now locked, so…