Add modern web UI with image uploads

[seanonymous]

Summary

• Add a Vite + TypeScript web UI with drag-and-drop image selection, upload flow, preview, controls, and status polling.
• Add Flask JSON/upload endpoints for listing options, storing validated input images, and serving input previews.
• Make local asset paths platform-neutral and include a default brush so the app can start cleanly on Linux.

Test plan

• python -m py_compile pyautopainter.py
• npm run build
• Smoke-tested /, /api/options, and a multipart image upload against the Flask app.

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