Make graph zoomable, pop-out-able, copyable

[musicog]

As of e6dd4, when the mermaid.js graph grows too large to fit its container, it no longer shrinks; rather it overflows, which is handled by scrolling.

It would be nice to introduce buttons for zooming in/out and for popping out a larger modal display as a bigger container for the graph on request.

E.g., icons like

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