T-76: Diagram text is the same size in every diagram

[nethum529]

Intent

The diagram block from #133 scales its SVG to fill the document column, so a tall diagram gets scaled up and a short one gets scaled down. Two diagrams in the same document then show text at very different sizes: a three node flow reads at about 18px and an eight node flow at about 28px. A document with several diagrams looks unplanned.

Given / When / Then

• Given two diagrams of different size in the same document

• Then the node text in both renders at the same size on screen

• Given a diagram wider than the document column

• Then it scales down to fit, and the whole diagram stays visible

• Given a diagram narrower than the document column

• Then it does not stretch to fill the width; it stays at its natural size and sits centred

• Given a very large diagram scaled down to fit

• Then the text is still readable, or the block says the diagram is too large to show at a readable size

• Given the light and dark themes

• Then nothing about the scale changes

Notes

• Fix the text size, then lay out around it. Do not lay out at an arbitrary scale and stretch the result.
• Follow-up to T-69: Built-in diagram block that lays itself out from nodes and edges #133. Same rules: no dependency, works offline, the layout stays a simple layered flow.