feat(ui): 添加 IDE 组件（文件浏览器 + 代码编辑器）

- 新增 CodeEditor 组件，基于 CodeMirror 实现多标签代码编辑
- 新增 FileExplorer 组件，支持文件树展开/折叠和文件选择
- 新增 IDE 组件，整合文件浏览器和代码编辑器
- 新增 SessionPanel 组件，用于会话管理
- 添加文件写入 API（PUT /api/files/write）
- 优化布局：IDE 始终显示，移除文件切换按钮
- 工作目录路径显示在文件浏览器标题栏，支持悬浮显示完整路径

packages/server/src/routes/files.ts

@@ -5,7 +5,7 @@
  */
 
 import { Hono } from 'hono';
-import { readdir, stat, readFile } from 'node:fs/promises';
+import { readdir, stat, readFile, writeFile, mkdir } from 'node:fs/promises';
 import { join, resolve, basename, extname, dirname } from 'node:path';
 import { searchFiles as coreSearchFiles, type FileIndexEntry } from '@ai-assistant/core';
 
@@ -398,4 +398,50 @@ filesRouter.get('/search', async (c) => {
   }
 });
 
+// ============================================================================
+// PUT /api/files/write - 写入文件内容
+// ============================================================================
+filesRouter.put('/write', async (c) => {
+  try {
+    const body = await c.req.json();
+    const { path: requestedPath, content } = body;
+
+    if (!requestedPath) {
+      return c.json({ success: false, error: 'Path is required' }, 400);
+    }
+
+    if (typeof content !== 'string') {
+      return c.json({ success: false, error: 'Content must be a string' }, 400);
+    }
+
+    const absolutePath = safePath(requestedPath);
+    if (!absolutePath) {
+      return c.json({ success: false, error: 'Access denied: path outside working directory' }, 403);
+    }
+
+    // 确保父目录存在
+    const parentDir = dirname(absolutePath);
+    await mkdir(parentDir, { recursive: true });
+
+    // 写入文件
+    await writeFile(absolutePath, content, 'utf-8');
+
+    const stats = await stat(absolutePath);
+    const name = basename(absolutePath);
+
+    return c.json({
+      success: true,
+      data: {
+        path: requestedPath,
+        name,
+        size: stats.size,
+        modified: stats.mtime.toISOString(),
+      },
+    });
+  } catch (error) {
+    const message = error instanceof Error ? error.message : 'Unknown error';
+    return c.json({ success: false, error: message }, 500);
+  }
+});
+
 export { filesRouter };