Config-driven theming

When rendering/layout is affected by configuration or theming, avoid magic numbers and ad-hoc config handling. Instead:

• Use a centralized “effective config” helper for all config-derived decisions, including correct type coercion (e.g., treat "false" the same as false).
• Derive theme-/appearance-sensitive values from themeVariables (or CSS vars like --mermaid-font-family) rather than hard-coding constants that break with different fonts/sizes.
• Apply theming values consistently across SVG/CSS/labels so users can rely on a single source of truth.
• If changing defaults would cause visual regressions, preserve existing behavior in the default path and gate improvements behind theme/config changes.

Example pattern:

// config.js
export function getEffectiveFlowchartHtmlLabels(siteConfig) {
  // Correctly coerce string booleans
  const v = siteConfig?.flowchart?.htmlLabels;
  return v === true || v === 'true';
}

And for theme-derived styling:

// instead of spacing = 60; or font-family: ${options.fontFamily};
const spacing = themeVariables?.noteSpacing ?? 60; // or derive from other theme vars
const fontFamily = 'var(--mermaid-font-family), trebuchet ms, verdana, arial, sans-serif';