Add static test UI for manual QA

Single-page vanilla HTML/JS frontend (register/login, balance,
place bet, withdraw) served by FastAPI at the same origin so it can
exercise the live API without CORS setup. Manual-testing aid only,
not part of the MVP spec.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/main.py

@@ -2,6 +2,7 @@ import asyncio
 from contextlib import asynccontextmanager
 
 from fastapi import FastAPI
+from fastapi.staticfiles import StaticFiles
 
 import app.bets.confirmation  # noqa: F401  (registers the "bet" confirmation handler)
 import app.rounds.confirmation  # noqa: F401  (registers the "payout" confirmation handler)
@@ -61,3 +62,6 @@ app.include_router(admin_router)
 @app.get("/health")
 async def health() -> dict[str, str]:
     return {"status": "ok"}
+
+
+app.mount("/", StaticFiles(directory="app/static", html=True), name="static")