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/queue.js

@@ -1,7 +1,7 @@
 // The incident queue: filter chips and a list of incident rows.
 
 import * as api from './api.js';
-import { h, clear, badge, severityBadge, emptyState, spinner } from './ui.js';
+import { h, clear, badge, severityBadge, emptyState, spinner, fadeOnOverflow } from './ui.js';
 import { ago, until, isFuture, severityClass, labelSummary } from './format.js';
 import { state, myID, onTeamChange } from './state.js';
 import * as onboarding from './onboarding.js';
@@ -118,6 +118,10 @@ function chipCount(id) {
   return null;
 }
 
+// Set the first time the chips are drawn: the strip element is the same one
+// for the life of the page, so one scroll listener and one observer will do.
+let chipsFade;
+
 function renderChips() {
   const el = document.getElementById('queue-filters');
   const chips = FILTERS.map((f) => {
@@ -135,9 +139,9 @@ function renderChips() {
   // There are no team chips here: the team selector in the sidebar (and in the
   // phone's top bar) is the one place the team is chosen, and chips for it
   // would be the same choice offered twice.
-  chips.push(h('span', { class: 'chips-fade', 'aria-hidden': 'true' }));
-
   clear(el, chips);
+  chipsFade ??= fadeOnOverflow(el);
+  chipsFade();
 }
 
 function renderList() {