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

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+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.

docs/roadmap/backlog/BT-005-defer-entry-spot-derivation.yaml

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+id: BT-005
+title: Defer Entry Spot Derivation to Backtest Run
+status: backlog
+priority: P1
+effort: S
+depends_on: []
+tags: [backtesting, ux, performance]
+summary: Only derive entry spot from historical data when user clicks Run, not on every form change.
+acceptance_criteria:
+  - Entry spot is NOT fetched when user changes start/end dates.
+  - Entry spot is NOT fetched when user changes other form fields.
+  - Entry spot IS derived when user clicks the Run button.
+  - Form remains responsive during date/field changes (no API calls blocking).
+  - Clear loading indicator shows when entry spot is being fetched during run.
+  - Previous entry spot value is retained until new one is derived.
+notes:
+  Current behavior calls derive_entry_spot on every date change which causes
+  API errors if user is still configuring other fields.
+  
+  The refresh_workspace_seeded_units function should not be called on date changes.
+  Entry spot derivation should happen inside start_backtest or as a separate
+  explicit "fetch spot" button if user wants to preview.