Migrate icon database maintenance to a JSON-first workflow (#4552)

Make WebBasedData/screenshot-database-v2.json the source of truth for icon and screenshot data. Remove the legacy XLSX source and the checked-in invalid URL snapshot, rewrite scripts/update-icons.ps1 around JSON validation and count synchronization, and fix its fallback behavior when invalid_urls.txt is absent.

Update the icon database contents by repairing truncated package names, adding explicit Devolutions WinGet entries, cleaning up obvious dead URLs, and refreshing stored package counts. Also add scripts/export-icon-database-gallery.ps1 to generate filtered Markdown galleries for visual review of icons and screenshots.

Author: mamoreau-devolutions