Adds basic tooltip integration.

[johnnymatthews]

Closes #350

Here's how it works, currently anyway:

The glossary stays in one place, data/glossary.json, and everything reads from there. When Zola builds the site, a small template partial loads that file and drops it into every page as a plain JS object on window. So the terms and definitions ship with the HTML itself, no fetch calls, no extra build step, nothing to keep in sync. Add a term to the JSON and it just shows up everywhere on the next build.

On the page, one vanilla JS file does the rest. It walks the text inside the article, matches any glossary term with a single whole-word regex (longest first, so "Merkle Tree" beats "Merkle"), and wraps each hit in a span. It skips code blocks, links, headings, and mermaid diagrams so it never breaks anything. The tooltip itself is one shared div: hover, tap, or keyboard focus shows it, Escape or tapping away closes it, and it flips or clamps to stay on screen. All styling is CSS using the existing theme variables, so dark and light mode both work.