feat(record): delete dataset from the Hub, not just local disk

[ravediamond]

Problem

Delete Dataset in LeLab only wipes the local cache directory. If the dataset had already been pushed to the Hub, it's still there afterwards — you have to go delete the repo by hand on huggingface.co. There's no huggingface_hub.delete_repo() call anywhere in the codebase.

Fix

• DatasetInfoRequest gets an optional delete_from_hub flag.
• handle_delete_dataset calls huggingface_hub.delete_repo(repo_id, repo_type="dataset") when that flag is set, before removing the local copy. A missing Hub repo is treated as a no-op (nothing to delete); any other failure aborts before touching local disk.
• On the Upload page, the delete dialog now shows a "also delete from the Hub" checkbox when the dataset is Hub-hosted. Checking it requires typing the repo id to confirm, since this is irreversible — same pattern GitHub itself uses for repo deletion.

Testing

• Added unit tests in tests/test_record.py covering: hub delete called when requested, skipped by default, and local copy preserved if the Hub call fails.
• pytest (204 passed), ruff check, ruff format --check clean.
• tsc --noEmit, eslint, npm run build clean (one pre-existing react-hooks/exhaustive-deps warning on this file, unrelated to this change).
• Manually tested against a dataset pushed to the Hub: unchecked box only removes local disk copy (Hub repo untouched), checked box + typed confirmation removes both.

frontend/dist not touched — CI rebuilds it on merge.

Opening as draft since I'd like a look at the confirm-by-typing UX before marking ready — happy to simplify to a plain checkbox if that's overkill for this.