feat: browser UI at / for QED Math environment

[SourasishBasu]

Closes #5

Changes

• server/ui.html: self-contained two-panel interface served at GET /
  • Left panel: current problem display, proof textarea, Get Problem / Get Guidelines / Submit Proof / Reset buttons, score bar with feedback, state card (Status, Episode ID, Step Count, Attempt)
  • Right panel: live Current Observation (JSON) and timestamped Action History
  • Auto-resets on load; polls GET /state every 2s to keep episode and step count in sync
• server/app.py: adds GET / route serving ui.html via HTMLResponse

[rycerzes]

remove the validate-inference.yml changes