Serve the settings search index from the server as JSON

Move index building off the client so there is no client-side HTML fetching
or DOM parsing (resolves Codacy's variable-fetch and DOMParser flags on the
read-only, same-origin index build).

- Add GET /v3/settings/search-index (pages_v3.py): renders the settings
  partials server-side and extracts each field's anchor id, key, label,
  tooltip, and section with a small stdlib HTMLParser, then caches the result
  keyed on the installed-plugin set. Parsing the rendered HTML keeps anchor
  ids identical to the live DOM, so the index cannot drift.
- settings-search.js: buildIndex() now does a single fetch of the literal
  endpoint + .json(); removed the per-partial fetch loop, DOMParser, scanDoc,
  CORE_TABS, and the plugin-id allowlist. Search, keyboard nav, navigation,
  and the per-tab filter are unchanged.

Net: fewer requests and no client-side HTML parsing. Verified with a new
endpoint test in test_web_settings_ui.py (12 pass) and the headless-Chromium
test (tooltip, filter, search navigate + flash all green).

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

test/test_web_settings_ui.py

@@ -115,3 +115,28 @@ def test_display_tooltip_carries_rich_text(client):
     body = client.get("/v3/partials/display").get_data(as_text=True)
     assert 'id="setting-display-brightness"' in body
     assert "Recommended:" in body  # rich detail authored into a tooltip
+
+
+def test_search_index_endpoint(client):
+    """The server-built search index powers the global settings search."""
+    resp = client.get("/v3/settings/search-index")
+    assert resp.status_code == 200
+    data = resp.get_json()
+    assert isinstance(data, dict) and isinstance(data.get("fields"), list)
+    fields = data["fields"]
+    assert len(fields) >= 40, "expected the core settings fields to be indexed"
+
+    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"):
+        assert anchor in by_id, f"{anchor} missing from search index"
+        entry = by_id[anchor]
+        assert entry["label"], f"{anchor} has no label"
+        assert entry["help"], f"{anchor} has no tooltip help"
+        assert entry["tab"] and entry["tabLabel"]
+
+    # Every entry must carry a non-empty label and a stable anchor id.
+    assert all(f["label"] and f["anchorId"].startswith("setting-") for f in fields)
+    # Section context is captured for grouped fields (e.g. Display hardware).
+    assert by_id["setting-display-brightness"]["section"] == "Hardware Configuration"