Add risk-based position sizing (以损定仓) with per-open k resize.

Manual vs risk modes are exclusive; each open floors k to 1 decimal so estimated premium+fees stay within the loss budget.

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

frontend/src/api/client.ts

@@ -323,6 +323,13 @@ export type StrategySettings = {
   perp_qty_eth?: number;
   option_qty_eth?: number;
   show_manual_trade_buttons?: boolean;
+  sizing_mode?: "manual" | "risk_based";
+  risk_loss_mode?: "percent" | "absolute";
+  risk_loss_pct?: number;
+  risk_loss_usdt?: number;
+  risk_capital_source?: "trading_account" | "manual";
+  risk_manual_capital_usdt?: number;
+  risk_sizing_preview?: Record<string, unknown>;
   exchange?: string;
 };