[Feature]: 支持多栏的自由布局

### Problem / opportunity

目前是可以用左栏|中栏|右栏|的 三段式描述bitfun的布局，类似于vscode或者obsidian，以中间的agent对话为核心，导致agent对话不可以被隐藏或者剔除。不爽的点在于远程服务器的时候，或者使用shell、file viewer 作为一段时间的主要工作时，会导致无法形成

|目录|正文|

的布局，而只能形成

|目录|空白|正文|

的布局。

### Proposed behavior

希望可以更自由的组合多个模块，虽然以agent 对话为主

### Product surface

Desktop app

### Details, examples, or constraints

_No response_

The issue names no files, tests, or entry points. Start by locating the desktop app's layout implementation and understanding how the current three-column arrangement handles the agent conversation, directory, shell, and file viewer modules. Done should allow multiple modules to be combined freely, including a directory-and-content layout without an empty middle column, while keeping agent chat optional.

Written by the indexing model from the issue text.