feat: 新增聊天消息搜索功能

- 会话内搜索：header 加搜索按钮，展开搜索栏，结果列表显示在消息区上方，点击跳转到对应时间
- 全局消息搜索：会话列表搜索框新增消息模式切换按钮，搜索结果展示在会话列表下方，点击跳转到对应会话
- preload 暴露 chat.searchMessages IPC

electron/preload.ts

@@ -218,6 +218,8 @@ contextBridge.exposeInMainWorld('electronAPI', {
     getContacts: () => ipcRenderer.invoke('chat:getContacts'),
     getMessage: (sessionId: string, localId: number) =>
       ipcRenderer.invoke('chat:getMessage', sessionId, localId),
+    searchMessages: (keyword: string, sessionId?: string, limit?: number, offset?: number, beginTimestamp?: number, endTimestamp?: number) =>
+      ipcRenderer.invoke('chat:searchMessages', keyword, sessionId, limit, offset, beginTimestamp, endTimestamp),
     onWcdbChange: (callback: (event: any, data: { type: string; json: string }) => void) => {
       ipcRenderer.on('wcdb-change', callback)
       return () => ipcRenderer.removeListener('wcdb-change', callback)