feat: add customizable search relays setting

Replace hardcoded SEARCHABLE_RELAY_URLS with user-configurable search
relays stored in localStorage. Add SearchRelaysSetting UI in System
settings page with add/remove/reset functionality.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/i18n/locales/zh.ts

@@ -659,6 +659,7 @@ export default {
     'Hide indirect': '隐藏间接通知',
     'Copy note content': '复制笔记内容',
     'Video loop': '视频循环',
-    'Automatically replay videos when they end': '视频播放结束后自动重新播放'
+    'Automatically replay videos when they end': '视频播放结束后自动重新播放',
+    'Relays used for searching notes (NIP-50)': '用于搜索笔记的服务器 (NIP-50)'
   }
 }