feat(search): 引入拼音搜索

* add pinyin search

* style(highlight): keep theme-aware background

- restore theme-aware .highlight with --highlight-bg

- add highlight vars for light/dark

chore: load pinyin lib before script.js

- ensure pinyin-match.js loads before script.js to avoid runtime dependency issues

docs: add third-party notice for pinyin-match

- add placeholder notice; replace with upstream license after verification

---------

Co-authored-by: neo <neo@example.com>
Co-authored-by: Maintainer (PR Edit) <maintainer@local>

licenses/pinyin-match.NOTICE.md

@@ -0,0 +1,7 @@
+Third-party notice: pinyin-match
+
+- File: assets/pinyin-match.js (minified vendor script)
+- Source: To be confirmed; replace with upstream repo URL and exact license.
+- License: To be added (likely MIT). Include the full license text once verified.
+
+Vendored to enable offline builds. This notice will be replaced with the exact license details once confirmed.