Touch gestures: pull to check for new items, swipe between items

- Pull the item list down from its top: checks the feed (or every feed, on All
  Subscriptions) for new items, which arrive as they do from the scan button.
  overscroll-behavior keeps the browser's own pull-to-reload out of it.
- Swipe the item you are reading left for the next, right for the one before,
  or back to the list from the first. A vertical move is a scroll; something
  that scrolls sideways, or takes typing, keeps its own swipe.

Touch events only, so a mouse never sets them off.

Closes #22.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -16,6 +16,9 @@ The long form, with what was wrong before and how it was found, is in
   Classic and Modern (the existing dark and light). Each that comes both ways has its own Light,
   Dark or Auto setting; Classic and Paper come one way only, so that setting is hidden for them.
   A theme chosen before this carries over.
+- Touch gestures: pull the item list down from its top to check the feed for new items, and
+  swipe the item you are reading left for the next one and right for the one before, or back
+  to the list from the first.
 
 ### Changed