Keep the scroll fade at the screen edge on the queue's filter row

The fade that hints at more filters to the right was an element inside the
scrolling row, so it scrolled away with the chips instead of staying at the
edge; the comment on it claimed the opposite. It is now a mask on the strip
itself, applied only while there is more to scroll to (fadeOnOverflow in
ui.js sets data-more), so it stays put and disappears at the end instead of
dimming the last chip.

The Team and Admin tab strips had an always-on mask from the earlier
"Sources is cut off" fix, which dimmed their last tab even when fully
scrolled; they use the same mechanism now.

Web UI only: no endpoint or JSON shape changed, so nothing to mirror in
terdut-tui.

internal/web/static/js/ui.js

@@ -32,6 +32,21 @@ export function clear(el, ...children) {
   return el;
 }
 
+// Marks a horizontally scrolling strip with data-more while there is more of it
+// to the right, which the CSS turns into a fade on that edge. The fade is a
+// mask on the strip itself, so it stays put at the edge of the screen: a fade
+// drawn as a child of the scroller scrolls away with the content. Returns the
+// update function, for a strip whose contents change under it.
+export function fadeOnOverflow(el) {
+  const update = () => {
+    el.toggleAttribute('data-more', el.scrollLeft + el.clientWidth < el.scrollWidth - 1);
+  };
+  el.addEventListener('scroll', update, { passive: true });
+  if (typeof ResizeObserver === 'function') new ResizeObserver(update).observe(el);
+  update();
+  return update;
+}
+
 // Stroke icons, 24×24. Built as SVG nodes so the CSP needs no inline anything.
 const ICONS = {
   alertTriangle: ['M12 4l9.5 16.5h-19z', 'M12 10v4.5', 'M12 17.5h.01'],