refactor: add Effect trait and apply_effects infrastructure

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>

src/ui/mod.rs

@@ -1,5 +1,6 @@
 pub mod app;
 pub mod category_panel;
+pub mod effect;
 pub mod formula_panel;
 pub mod grid;
 pub mod help;