Add pan and zoom to DAG preview

[bentsherman]

Adds pan and zoom to the DAG preview webview.

The original version of this PR used svg-pan-zoom, which rewrites the SVG's viewBox and so needs a sized SVG to work against. That forced mermaid.render() instead of startOnLoad, a custom events handler to re-add dragging, and a hardcoded height: 1024px. It also predates the bundled Mermaid and the webview CSP, both of which it would now conflict with.

This version drops the library. The <pre class="mermaid"> goes inside a .viewport div (flex: 1; overflow: hidden) and pan/zoom is a CSS transform on the pre, driven by ~50 lines of vanilla JS. No dependency, no CSP change, startOnLoad untouched. The height is no longer fixed: the body is a flex column at 100vh and the viewport takes whatever is left.

Keeping the transform on the wrapper rather than the viewBox means Export as SVG still serializes the unzoomed diagram instead of baking in the current zoom level. Panning uses window listeners rather than setPointerCapture, which would retarget the subsequent click to the viewport and break click-to-open-file; a drag of more than 4px swallows the click so panning doesn't open files by accident.

Pan/zoom is webview-only, passed through htmlHead's extra parameter. The standalone HTML export is left alone since browsers already scroll and zoom it.

Controls: drag to pan, scroll to zoom at the cursor (clamped 0.1x-20x), double-click to reset.

[alvaromartmart]

Years ago I did a proof of concept of displaying nextflow DAG svg using OpenSeadragon, you can find it here. OpenSeadragon might be a bit overkill for this purpose (or not) but worth checking as the setup could be quite easy and it handles pan, zoom etc (even minimaps) out of the box.