Implement four-page NiceGUI app structure

[KelvinChueng]

Motivation

• Provide a simple multi-page UI so the application can host the requested sections: 首页, 图片管理, 工作流编辑页面, 设置页面.
• Keep the runtime configuration required for deployment with title AI绘图 and port 8080.

Description

• Refactored main.py into a multi-page NiceGUI app and added a shared top navigation via render_nav().
• Implemented separate route handlers for /, /images, /workflow, and /settings with basic placeholder content.
• Kept the app launch configuration as ui.run(title='AI绘图', port=8080).
• Added requirements.txt containing nicegui to declare the dependency.

Testing

• Ran python -m py_compile main.py which completed successfully.
• Attempted pip install -r requirements.txt but installation failed in this environment due to network/proxy restrictions (could not fetch nicegui).

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Codex Task

[KelvinChueng]

hebing