feat: add candlestick chart with portfolio value line (BT-004)

- Add spot_open field to BacktestDailyPoint for complete OHLC data
- Replace line chart with candlestick chart showing price OHLC
- Add portfolio value line on secondary Y-axis
- Add _chart_options_from_dict for rendering job results
- Update both render_result and render_job_result to use new chart

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

@@ -1,19 +0,0 @@
-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.