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/app.rs

@@ -91,6 +91,12 @@ impl App {
         }
     }
 
+    pub fn apply_effects(&mut self, effects: Vec<Box<dyn super::effect::Effect>>) {
+        for effect in effects {
+            effect.apply(self);
+        }
+    }
+
     /// True when the model has no categories yet (show welcome screen)
     pub fn is_empty_model(&self) -> bool {
         self.model.categories.is_empty()