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>

web/build.mjs

@@ -21,7 +21,7 @@ const here = path.dirname(fileURLToPath(import.meta.url));
 // The files are one script, concatenated in this order, not modules: they share one top-level
 // scope, as the single inline script did, and code that runs at load needs what came before it.
 const PAGES = {
-  'index.html': { script: 'app.js', src: ['util', 'theme', 'feeds', 'feedpage', 'items', 'player', 'dialogs', 'events'] },
+  'index.html': { script: 'app.js', src: ['util', 'theme', 'feeds', 'feedpage', 'items', 'player', 'dialogs', 'gestures', 'events'] },
   'login.html': { script: 'login.js', src: ['login'] },
 };