Add pinyin-search plugin

[RAMBOO1990]

Add pinyin-search plugin — enables Chinese pinyin search in KeeWeb.

This plugin overrides EntryModel.prototype._buildSearchText to append full pinyin
and first-letter pinyin to each entry's search text. Users can search by:

• Full pinyin: zhifubao → 支付宝, taobao → 淘宝
• First-letter pinyin: zfb → 支付宝, wx → 微信

Key details:

• Pinyin dictionary: 401 keys, ~8300 chars, from kMandarin_8105.txt (通用规范汉字表 2013)
• Dictionary is inlined in plugin.js — zero network requests at runtime
• Hosted on GitHub Pages: https://ramboo1990.github.io/keeweb-pinyin-search/