Blocks should have descriptive icons

It would be a huge improvement if blocks had descriptive icons that illustrated what they did. This is especially true when the tool is used to teach younger students for whom reading words like "rotate" is not quite second nature.

I could work on making these as images; but I've also just switched over the UI icons on the main site to https://materialdesignicons.com/ which has a lot of icons in SVG format that might be useful?

Start by reviewing the main site's existing UI icon integration and the block-rendering entry point; no source files or tests are named in the issue. Compare suitable Material Design Icons SVGs for the blocks, then confirm that each block has a recognizable descriptive icon and that the teaching UI remains usable.

Written by the indexing model from the issue text.