revert: restore server/app.py to last known working state

Deploy HTML landing page at root / (replaces JSON response)

revert: restore server/app.py to last working state

fix: remove openenv.core dependency — pure FastAPI, graceful fallback

fix: redirect root / to /ui so judges see Gradio UI not raw JSON

fix: pure FastAPI on port 7860 — all OpenEnv endpoints live + Gradio at /ui

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