refactor: deduplicate NAV layout setup into shared helper

[Copilot]

lagged_performance_plot and smoothed_holdings_performance_plot contained identical 27-line Plotly layout blocks differing only by title string.

Changes

• New _apply_nav_layout static method — encapsulates the shared update_layout / update_xaxes / update_yaxes / log_scale logic, parameterised by title and log_scale
• Both plot methods now delegate to the helper, replacing the duplicated blocks with a single call each

# Before (repeated verbatim in two methods)
fig.update_layout(
    title="NAV accumulated by lag",
    hovermode="x unified",
    plot_bgcolor="white",
    legend={...},
    xaxis={"rangeselector": {"buttons": [...]}, ...},
)
fig.update_yaxes(title_text="NAV (accumulated)")
fig.update_xaxes(showgrid=True, gridwidth=0.5, gridcolor="lightgrey")
fig.update_yaxes(showgrid=True, gridwidth=0.5, gridcolor="lightgrey")
if log_scale:
    ...

# After
self._apply_nav_layout(fig, title="NAV accumulated by lag", log_scale=log_scale)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Duplicated code?</issue_title>
<issue_description>Found in src\jquantstats\analytics_plots.py:257-283 , src\jquantstats\analytics_plots.py:592-618</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Duplicated code? #371

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