Point user-facing HLE links to public lastexam.ai

The cais/hle HuggingFace dataset is gated and returns 404 for visitors who haven't accepted its terms, so per-question links, the browse-page benchmark link, and the footers all 404'd for the public. Point them at the public HLE site (lastexam.ai) instead. README's HuggingFace links stay, since building the question bank genuinely requires accepting the dataset terms there.