Show a spinner while Step 4 calculates statistics - #9

Summary

Clicking Calculate statistics now shows an animated spinner immediately, with messages for reading the histogram table, calculating group statistics and drawing the distributions. The button says Calculating… and is disabled until the work finishes. Previously, Step 4 gave no feedback during calculation and announced completion before plotting finished.

The button is restored on success or failure. Read errors keep their existing message; unexpected calculation or plotting errors show an error card and record a traceback in Technical details. Repeated requests while busy are ignored.

Decisions

Reuse the existing notebook spinner and button styling. Document the interaction in the user guide and plan.

Notes for review

• Built on the pending high-priority fixes in Fix singleton-axis readiness and preserve outputs on failed updates #8. The Step 4 change is isolated in commit ebf501e; the earlier commit remains included until that work reaches main.
• Widgets tested headlessly; standalone HTML previews generated for loading, plotting and completion. Browser visual verification remains unavailable following the earlier automatic approval rejection of browser launch.
• The user's executed notebook and local AGENTS.md were preserved and excluded from the commit. The full suite ran in a clean temporary worktree with PYTHONPATH pointing to its source.

Tests

• Four new regression cases first failed without the change, then passed: spinner/button state throughout loading, pooling and plotting; duplicate-request prevention; recovery after read, calculation and plotting failures.
• Full suite on Windows / Python 3.12.11: 477 passed, 2 skipped.
• ruff check ., ruff format --check . and git diff --check pass.