Add Binance SIM market adapter and exchange switch in settings.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/app/api/market.py

@@ -14,8 +14,7 @@ router = APIRouter(prefix="/api/market", tags=["market"])
 async def market_snapshot(_user: Annotated[str, Depends(require_user)]) -> dict:
     gw = get_gateway()
     snap = gw.snapshot_dict()
-    if snap.get("pair") is None:
-        raise HTTPException(status_code=503, detail="market not aligned yet")
+    # 切换交易所后短时可能尚未对齐 ATM；仍返回结构便于前端展示交易所
     return snap