Address CodeRabbit review on settings search/tooltips

- pages_v3: include Durations tab in the search index so
  setting-durations-* fields are actually indexed
- test_web_settings_ui: assert setting-durations-clock is present in
  the search-index endpoint response
- settings-search.js: on index fetch failure, reset buildPromise
  instead of caching an empty (truthy) index so search can retry
- settings-search.js: filterScope returns null (not document) when no
  tab container matches, and the caller guards, so the per-tab filter
  can't hide fields across unrelated tabs
- settings-search.js: refresh the stale header comment to describe the
  server-side JSON index flow
- app.css: cap #settings-search-results height with overflow-y so the
  dropdown scrolls instead of overflowing small screens

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014gZxznuxw8L92FUMBN3Nqz

web_interface/static/v3/app.css

@@ -838,6 +838,8 @@ button.bg-white {
     box-shadow: var(--shadow-lg);
     z-index: 50;
     padding: 0.25rem;
+    max-height: min(60vh, 24rem);
+    overflow-y: auto;
 }
 
 .ssr-group {