docs: download-all in changelog, help page, README and llms.txt

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BNhLunz586g6DeYMa7Qsd1

CHANGELOG.md

@@ -12,6 +12,12 @@ All notable changes to Klarbild are documented here. Newest first.
   off entirely, and pages and files are noindex. Access is bound to the link, not the
   image: swapping in another item id on a valid link returns 404. Private-session
   results can never be shared.
+- **Download everything at once**: a share with several pictures gets one button above the
+  grid for a ZIP at full resolution. Written to stream, not buffer -- one file at a time,
+  waiting for archiver's `entry` before reading the next. Measured: 440 MB of images, RSS
+  peaked at 196 MB against a 120 MB baseline. Capped at 200 files and 500 MB, past which
+  the archive closes with a note saying what happened. Respects the download lock and the
+  passphrase; rate limited per link and address.
 - **Passing it on** by e-mail (several recipients, no attachment — a link can be
   withdrawn later, an attachment cannot) or via `wa.me`, which opens WhatsApp with the
   text prepared. The WhatsApp Business API needs a Meta account, approved templates and