Expand Mermaid diagrams into a lightbox on click

[jwils]

Why

Mermaid diagrams render constrained to the content column, so anything beyond a trivial flowchart is too small to read.

What

• Clicking a rendered diagram (or the expand button that appears on hover) opens it in a native <dialog> sized to the viewport, with the SVG scaled to fit
• Click anywhere or press Escape to close; the dialog is removed on close and tagged data-turbo-temporary so Turbo never caches it
• Extend the foreignObject line-height fix (Fix clipped mermaid node labels by pinning label line-height #149) to the lightbox, which lives outside .markdown-rendered
• System spec covering expand and click-to-close

Risk Assessment

Low — additive UI behavior scoped to rendered Mermaid diagrams; no server-side changes.

Screenshots

Inline diagram with the expand affordance on hover:

Clicking opens the lightbox, scaled to the viewport:

For comparison, the same diagram inline before expanding:

[HamptonMakes]

Please write system specs for this!