Generate backend topic distribution charts

[nateafish]

Summary

• add a reusable matplotlib helper and API endpoint to export topic/area distribution charts as PNG/SVG assets
• surface the backend-rendered topic and area charts in the dashboard with download links instead of client-side Plotly bars
• style the new category distribution panels to match the existing visualization layout

Testing

• python -m py_compile 重分类gui.py

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https://chatgpt.com/codex/tasks/task_e_68f7a044c3148327867d44a6ffaa03ab