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/blueprints/pages_v3.py

@@ -232,6 +232,7 @@ def settings_search_index():
     core_tabs = [
         ('general', 'General', _load_general_partial),
         ('display', 'Display', _load_display_partial),
+        ('durations', 'Durations', _load_durations_partial),
         ('schedule', 'Schedule', _load_schedule_partial),
         ('wifi', 'WiFi', _load_wifi_partial),
     ]