Swizzled @docusaurus/theme-mermaid's Mermaid component to wrap
every diagram with a portal-based lightbox overlay.
An expand icon appears on diagram hover. Clicking opens the SVG
in a full-screen overlay (90vw × 85vh, scrollable). Closes via
backdrop click, Escape key, or close button. SSR-safe, no
external dependencies. Matches Tibber electric color theme.
Impact: Users can inspect complex flowcharts (e.g. the Options
Flow wizard) without squinting at small embedded diagrams.
