feat(ui): add buffers HashMap for text input state management

Introduce a buffers HashMap to manage text input state across different
modes (command, edit, formula, category, export).

Changes:
- Added buffers field to GridWidget and updated constructor
- Updated draw_command_bar to use app.buffers instead of mode buffer
- Updated grid edit indicator to use buffers HashMap
- Added tests for command mode buffer behavior:
  * command_mode_typing_appends_to_buffer
  * command_mode_buffer_cleared_on_reentry

Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/Qwen3.5-35B-A3B-GGUF:Q5_K_M)

src/ui/app.rs

@@ -290,4 +290,39 @@ mod tests {
             "mode must not be Normal after import wizard is opened"
         );
     }
+
+    #[test]
+    fn command_mode_typing_appends_to_buffer() {
+        use crossterm::event::KeyEvent;
+        let mut app = two_col_model();
+        // Enter command mode with ':'
+        app.handle_key(KeyEvent::new(KeyCode::Char(':'), KeyModifiers::NONE))
+            .unwrap();
+        assert!(matches!(app.mode, AppMode::CommandMode { .. }));
+        assert_eq!(app.buffers.get("command").map(|s| s.as_str()), Some(""));
+
+        // Type 'q'
+        app.handle_key(KeyEvent::new(KeyCode::Char('q'), KeyModifiers::NONE))
+            .unwrap();
+        assert_eq!(app.buffers.get("command").map(|s| s.as_str()), Some("q"));
+    }
+
+    #[test]
+    fn command_mode_buffer_cleared_on_reentry() {
+        use crossterm::event::KeyEvent;
+        let mut app = two_col_model();
+        // Enter command mode, type something, escape
+        app.handle_key(KeyEvent::new(KeyCode::Char(':'), KeyModifiers::NONE))
+            .unwrap();
+        app.handle_key(KeyEvent::new(KeyCode::Char('x'), KeyModifiers::NONE))
+            .unwrap();
+        assert_eq!(app.buffers.get("command").map(|s| s.as_str()), Some("x"));
+        app.handle_key(KeyEvent::new(KeyCode::Esc, KeyModifiers::NONE))
+            .unwrap();
+
+        // Re-enter command mode — buffer should be cleared
+        app.handle_key(KeyEvent::new(KeyCode::Char(':'), KeyModifiers::NONE))
+            .unwrap();
+        assert_eq!(app.buffers.get("command").map(|s| s.as_str()), Some(""));
+    }
 }