Show a Calculating indicator in the app's status bar while the pivot
rebuilds, since Perspective's own beaker is only rendered while the
settings panel is open. Also make the light/dark toggle actually reach
the pivot: the theme is a ViewerConfig field, not an attribute, and
stored layouts were reasserting their author's theme on every restore.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>