Function render_html_payload 

pub fn render_html_payload(
    overview: &OverviewResult,
    projects: &ProjectResult,
    trend: &TrendResult,
    sessions: &[SessionData],
    calc: &PricingCalculator,
    wrapped: Option<&WrappedResult>,
    active_session_id: Option<&str>,
) -> String

Build the unified HTML report payload.

Reuses existing render_*_json functions for overview/projects/trend, then builds session summaries and heatmap data directly from SessionData.