refactor(ui): improve layout management and record sharing in App

Refactor App and DrillState to use Rc for sharing records, and introduce
rebuild_layout() to manage GridLayout lifecycle. This ensures the layout is
consistently updated when the model or drill state changes.

- Use Rc<Vec<(CellKey, CellValue)>> for DrillState.records to allow cheap
  cloning
- Add layout field to App
- Implement rebuild_layout() in App to refresh the GridLayout
- Update App::new and App::cmd_context to use the new layout management
- Ensure layout is rebuilt after applying effects and handling key events

Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/gemma-4-26B-A4B-it-GGUF:UD-Q5_K_XL)

src/ui/effect.rs

@@ -398,7 +398,7 @@ pub struct StartDrill(pub Vec<(CellKey, CellValue)>);
 impl Effect for StartDrill {
     fn apply(&self, app: &mut App) {
         app.drill_state = Some(super::app::DrillState {
-            records: self.0.clone(),
+            records: std::rc::Rc::new(self.0.clone()),
             pending_edits: std::collections::HashMap::new(),
         });
     }