feat: add option contracts to overview, fix default dates, add roadmap items

- Move option contracts from daily results table to overview cards (constant throughout backtest)
- Fix default dates to March 2026 (2026-03-02 to 2026-03-25)
- Add BT-004 backlog item: candlestick chart with portfolio value line on secondary axis
- Add BT-005 backlog item: defer entry spot derivation to backtest run (not on every date change)

docs/roadmap/backlog/BT-004-backtest-visualization-chart.yaml

@@ -0,0 +1,19 @@
+id: BT-004
+title: Backtest Visualization Chart
+status: backlog
+priority: P1
+effort: M
+depends_on:
+  - BT-001
+tags: [backtesting, visualization, charts]
+summary: Add interactive candlestick chart showing price OHLC with portfolio value line overlay.
+acceptance_criteria:
+  - Candlestick chart displays OHLC data (open, high, low, close) for each day.
+  - Portfolio value (underlying + option) shown as line on secondary Y-axis.
+  - Chart updates when backtest results change.
+  - Chart is responsive and readable on mobile/tablet viewports.
+  - Chart library matches existing app styling (dark mode support).
+notes: |
+  Consider using a lightweight charting library that integrates well with NiceGUI.
+  Portfolio value line should be clearly distinguishable from candlesticks.
+  May want to add toggle for showing/hiding different series.