show a spinner + message in the report during evaluatoin

When a user is writing a report with pyodide, they currently pay a penalty on pageload. There are two vectors for improving the experience: (1) speed up the pyodide stack loading and (2) give some feedback (spinner + message) so a user is not wondering what this blank page is.

Start by locating the report evaluation entry point and the pyodide loading path; the issue does not name specific files or tests. Confirm the loading state is visible during evaluation instead of a blank page, with a spinner and explanatory message, and check the existing report flow for validation.

Written by the indexing model from the issue text.