UI tweaks to cell creation

[kasbah]

• Doesn't create an empty "stem" cell when you unfocus without selecting anything
• Newly created cells grab focus

[epatters]

I'm afraid this one doesn't seem to work properly for me (running frontend locally, on Firefox, rebased off main). When I press Shift+Enter with the active cell a stock at the top of a stock-flow notebook, the cell constructor menu appears at the end of the notebook.

Beyond that, it seems that this PR effectively removes stem cells from the notebook in that it is no longer possible to create them. I'm not committed to the stem cell concept (which was modeled on Notion). However, if we are going to replace it, we should go further and remove it as fully as practical from the codebase, though a complete removal would require a migration filtering notebook cells, which is annoying. In its current form, this PR seems to leave significant vestigial code.