Define Effect trait in ui/effect.rs with concrete effect structs for
all model mutations, view changes, navigation, and app state updates.
Each effect implements apply(&self, &mut App). Add App::apply_effects
to apply a sequence of effects. No behavior change yet — existing
key handlers still work as before.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Removes dead use statements across dispatch, formula, import, model, and
UI modules. Prefixes intentionally unused variables with _ in app.rs,
analyzer.rs, and grid.rs.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
