feat: add Plotly PnL chart to CLI

[Satvik-Singh192]

Description

This PR adds an interactive Plotly HTML chart to the CLI backtest command, providing users with enhanced visualization capabilities while maintaining full backward compatibility with the existing matplotlib PNG output.

Key Changes:

• Added create_equity_curve_plot function in new metrics/plotting.py module
• Integrated new --plotly flag into CLI backtest command
• Generated Plotly HTML charts are saved to output/backtest_plot.html
• Maintained existing matplotlib PNG functionality as default behavior
• Added comprehensive unit smoke tests for Plotly HTML file creation

Testing

• pytest passes locally
• Linting passes (ruff check src/ tests/ - no errors in new code)

Semver Changes

• Patch (bug fix, no new features)
• Minor (new features, no breaking changes)
• Major (breaking changes)

Issues

Closes #5 - "Add Plotly PnL chart to CLI"

Checklist

• I have read the Contributing Guidelines.

Additional Notes

• Plotly is an optional dependency (already listed in project README)
• Users can generate both PNG and HTML charts using --plotly flag
• Interactive charts include hover tooltips, zoom/pan capabilities, and performance metrics
• Backward compatibility ensures existing workflows remain unchanged
• Unit tests verify HTML file creation and basic functionality