Function html 

pub fn html(
    steps: &[Step],
    totals: &SessionTotals,
    no_cost: bool,
    annotations: Option<&Annotations>,
) -> String

Render a session as self-contained HTML with inline CSS, no JS, no external assets. Color palette mirrors the TUI — cyan/user, green/assistant, yellow/tool_use, magenta/tool_result. When annotations is provided, steps with a note render the text in a magenta-bordered <div class="note"> below the meta line.