feat: enhanced paper trading reset with capital and reserve controls

- Add initial capital input (toggle between broker balance or custom amount)
- Add reserve/active pool split slider (0-50%, default 20%)
- Backend accepts reserve_pct in reset request body
- Note in UI that Alpaca balance reset requires Alpaca dashboard
- Confirmation dialog shows exact capital and split being applied

frontend/src/api/tradingHooks.ts

@@ -316,9 +316,12 @@ export function useBacktestLaunch() {
 export function useResetPaperTrading() {
   const qc = useQueryClient();
   return useMutation({
-    mutationFn: (initial_capital: number = 0) =>
+    mutationFn: (params: { initial_capital?: number; reserve_pct?: number } = {}) =>
       apiPost<{ reset: boolean; initial_capital: number; active_pool: number; reserve_pool: number; broker: Record<string, number> }>(
-        'trading', '/api/trading/reset', { initial_capital },
+        'trading', '/api/trading/reset', {
+          initial_capital: params.initial_capital ?? 0,
+          reserve_pct: params.reserve_pct ?? undefined,
+        },
       ),
     onSuccess: () => {
       qc.invalidateQueries({ queryKey: ['trading-status'] });