New tool: Remove image background, on the device #44

What

A Remove Background tool: add a photo (a person, a product, a signature) and get a PNG with a transparent background, or a plain white or coloured one. Everything runs on the device, and the photo never leaves it.

Why

It's one of the most searched image tasks (passport photos, product shots, profile pictures, signatures for forms). Popular sites upload your photo and limit free use.

Ideas

• A small segmentation model run in the browser with ONNX Runtime Web (MIT) or TensorFlow.js (Apache-2.0). Portrait or salient-object models such as MODNet or U²-Net-lite are a few MB.
• The model must be self-hosted under /models/ like the OCR files (see vite-plugins/ocr-assets.ts): the page may only load files from its own origin. It must also be permissively licensed, so please state the model's licence in the proposal.
• Load the model only when this tool runs, as OCR does, so the rest of the site stays tiny.
• Nice extras: a before/after slider, a background colour picker (white for forms), and an edge-softness slider.
• Signature cleanup is a quick first step that needs no model: keep dark ink and make light paper transparent (a threshold slider). It would be very useful for forms.

Steps

1. Comment with the model and runtime you propose, and their licences and download size.
2. Add src/engine/background.ts, a tool entry (format: 'image') in src/site.ts, and strings in src/i18n.ts.
3. Add e2e tests. A simple synthetic image (a dark shape on a light background) is enough to check transparency in the output.