feat(ui): 实现 @ 文件提及自动补全功能

- Core: 添加 file-index 模块，使用 ripgrep 索引文件，fuzzysort 模糊搜索
- Server: 添加 /api/files/search 端点，支持文件模糊搜索
- Server: WebSocket 消息处理中将 @filepath 转换为 ./filepath 格式
- UI: 新增 FileMenu 组件，显示文件搜索结果列表
- UI: 新增 FileMentionTag 组件，高亮显示文件提及
- UI: 新增 useFileMention hook，管理文件提及状态
- UI: ChatInput 集成 @ 触发的文件自动补全
- UI: ChatMessage 用户消息中高亮显示 @filepath

packages/server/src/routes/files.ts

@@ -7,6 +7,7 @@
 import { Hono } from 'hono';
 import { readdir, stat, readFile } from 'node:fs/promises';
 import { join, resolve, basename, extname, dirname } from 'node:path';
+import { searchFiles as coreSearchFiles, type FileIndexEntry } from '@ai-assistant/core';
 
 const filesRouter = new Hono();
 
@@ -368,4 +369,33 @@ filesRouter.get('/tree', async (c) => {
   }
 });
 
+// ============================================================================
+// GET /api/files/search?query=&limit=&type= - 模糊搜索文件
+// ============================================================================
+filesRouter.get('/search', async (c) => {
+  const query = c.req.query('query') || '';
+  const limit = parseInt(c.req.query('limit') || '10', 10);
+  const type = (c.req.query('type') || 'file') as 'file' | 'directory' | 'all';
+
+  try {
+    const results = await coreSearchFiles({
+      query,
+      limit,
+      type,
+      cwd: workingDirectory,
+    });
+
+    return c.json({
+      success: true,
+      data: {
+        files: results,
+        total: results.length,
+      },
+    });
+  } catch (error) {
+    const message = error instanceof Error ? error.message : 'Unknown error';
+    return c.json({ success: false, error: message }, 500);
+  }
+});
+
 export { filesRouter };