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

test/test_web_settings_ui.py

@@ -129,7 +129,7 @@ def test_search_index_endpoint(client):
     by_id = {f["anchorId"]: f for f in fields}
     # Representative fields across tabs must be present with usable text.
     for anchor in ("setting-general-timezone", "setting-display-brightness",
-                   "setting-wifi-password"):
+                   "setting-wifi-password", "setting-durations-clock"):
         assert anchor in by_id, f"{anchor} missing from search index"
         entry = by_id[anchor]
         assert entry["label"], f"{anchor} has no label"