docs(aggregation): Add hosted interactive plotter webapp

[KhusPatel4450]

Adds a self-contained Dash webapp in visualization/ that hosts the interactive gradient plotter, and a GitHub Actions workflow that auto-deploys it to Hugging Face Spaces on every push to main.

• visualization/app.py, Dash app refactored from tests/plots/interactive_plotter.py
• visualization/_utils.py, plotting utilities (copy of tests/plots/_utils.py)
• visualization/requirements.txt + visualization/Dockerfile, for HF Spaces (Docker SDK)
• visualization/README.md, HF Spaces metadata
• .github/workflows/deploy-visualization.yml, deploys visualization/ to HF Spaces on push to main

The app accepts query parameters so individual aggregator pages can embed it with a pre-selected method (as suggested by @PierreQuinton)

Before we can deploy we need to do this:

1. Create a Hugging Face Space at torchjd/interactive-plotter with Docker as the SDK
2. Add HF_TOKEN (an HF token with write access to that Space) as a GitHub Actions secret

Once those are in place, any push to main that touches visualization/ will redeploy automatically

[ValerianRey]

Thank you very much for taking this! Could you deploy it on under some random name on HF so that we it already?