refactor: remove confusing benchmark and date range controls

Removed S&P 500 benchmark checkbox and date range selector from Dashboard
as they would cause UX confusion:

- Benchmark: Individual stock lines don't benefit from S&P 500 comparison
- Date range: Would only affect one chart while metrics use full year data

Future enhancement: Add separate chart comparing aggregate portfolio
performance vs S&P 500 if benchmark comparison is needed.

app.py

@@ -2029,29 +2029,7 @@ def create_interface() -> gr.Blocks:
                         with gr.Column(scale=1, min_width=400):
                             risk_plot = gr.Plot(label="Risk Metrics Dashboard", container=True)
 
-                    # Middle row - Performance controls and chart
-                    with gr.Row():
-                        date_range_selector = gr.Dropdown(
-                            choices=[
-                                ("1 Month", "1M"),
-                                ("3 Months", "3M"),
-                                ("6 Months", "6M"),
-                                ("1 Year", "1Y"),
-                                ("Year to Date", "YTD"),
-                                ("All", "ALL"),
-                            ],
-                            value="1Y",
-                            label="Time Period",
-                            scale=1,
-                            elem_id="date-range-selector"
-                        )
-                        benchmark_toggle = gr.Checkbox(
-                            label="Show S&P 500 Benchmark",
-                            value=False,
-                            scale=1,
-                            elem_id="benchmark-toggle"
-                        )
-
+                    # Middle row - Performance chart
                     with gr.Row():
                         performance_plot = gr.Plot(label="Historical Performance", container=True)