Swiping past the 50th item fetches the next ones (#383)

The item list is fetched 50 at a time, and only the list's Load more
button asked for more. A swipe or j stops at the last item loaded, so
after 50 items the reader bounced on every swipe until a reload brought
a fresh page of unread ones.

Turning to within five of the end now fetches the next page behind. Its
offset counts what is still unread on the Unread tab: the items read
here have left the server's unread list, and Load more's flat 50 a page
skipped as many unread items as had been read. A refresh (a return from
the background, a live update) always asks for the first page, as the
kept list it fills is one, where it used to ask for whatever page Load
more had last reached. A page appended after new items arrived no longer
repeats ones already loaded.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Fixed
+- Swiping, or j, through more than 50 items no longer stops at the 50th until a reload: the next ones are fetched as you near the end, and Load more on the Unread tab no longer skips as many unread items as you had read (#383).
 - ipx-web no longer logs a deprecation warning about its metrics setting at every start (#381).
 - A tap's time in the traces ends with the last answer to what it set off, not with a refresh made seconds later (#379).
 - Switching away from the page a minute after using it no longer stops it with "Maximum call stack size exceeded" (#378).