fix: portfolio KeyError and deprecated matplotlib style

- Use .get() for total_current_value to prevent KeyError when metrics are incomplete
- Replace deprecated "seaborn" style with "default" (removed in matplotlib 3.6+)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: rpwallin

maverick_mcp/api/routers/portfolio.py

@@ -906,7 +906,7 @@ def get_my_portfolio(
                 "positions": positions_list,
                 "metrics": {
                     "total_invested": metrics["total_invested"],
-                    "total_current_value": metrics["total_current_value"],
+                    "total_current_value": metrics.get("total_current_value", 0),
                     "total_unrealized_gain_loss": metrics["total_unrealized_gain_loss"],
                     "total_return_percent": metrics["total_return_percent"],
                     "number_of_positions": len(positions_list),

maverick_mcp/backtesting/visualization.py

@@ -19,7 +19,7 @@ def set_chart_style(theme: str = "light") -> None:
     Args:
         theme (str): Chart theme, either 'light' or 'dark'
     """
-    plt.style.use("seaborn")
+    plt.style.use("default")
 
     if theme == "dark":
         plt.style.use("dark_background")