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

services/trading/app.py

@@ -68,6 +68,7 @@ class CapitalRequest(BaseModel):
     """Body for POST /api/trading/reset."""
 
     initial_capital: float = 0.0
+    reserve_pct: float | None = None
 
 
 class BacktestRequest(BaseModel):
@@ -430,7 +431,8 @@ async def reset_paper_trading(body: CapitalRequest) -> dict[str, Any]:
     else:
         capital = 100_000.0
 
-    reserve_pct = engine.config.reserve_siphon_pct
+    reserve_pct = body.reserve_pct if body.reserve_pct is not None else engine.config.reserve_siphon_pct
+    reserve_pct = max(0.0, min(1.0, reserve_pct))  # clamp 0-100%
     reserve = capital * reserve_pct
     active = capital - reserve