10,810 problems, referrals, and inline syntax checking

What changed

• 5,296 new problems landed, bringing the bank to 10,810, and curated roadmap tracks returned to the main practice page.
• Inline syntax error checking arrived in every IDE on the site, while AI analysis gained percentile rankings and a test-pass-rate graph.
• A referral program (invite five friends, get a month of Ultimate free), a ban and suspension system with automatic detection, and a rate-limiting and authentication hardening pass rounded out the batch.