refactor(ui): simplify panel mode mapping and add scroll tests

Adds a `mode()` method to the `Panel` enum to map panels to their
corresponding `AppMode`. Simplifies `TogglePanelAndFocus` in `cmd.rs`
to use this method instead of a manual match block.

Also adds regression tests in `app.rs` to verify that viewport
scrolling now correctly handles small terminal heights.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (unsloth/gemma-4-31B-it-GGUF:UD-Q5_K_XL)

src/ui/app.rs

@@ -482,6 +482,56 @@ mod tests {
         assert_eq!(app.model.active_view().selected.0, 5);
     }
 
+    #[test]
+    fn jump_last_row_scrolls_with_small_terminal() {
+        let mut app = two_col_model();
+        // Total rows: A, B, C + R0..R9 = 13 rows. Last row = 12.
+        for i in 0..10 {
+            app.model.category_mut("Row").unwrap().add_item(&format!("R{i}"));
+        }
+        app.term_height = 13; // ~5 visible rows
+        app.model.active_view_mut().selected = (0, 0);
+        // G jumps to last row (row 12)
+        app.handle_key(KeyEvent::new(KeyCode::Char('G'), KeyModifiers::NONE))
+            .unwrap();
+        let last = app.model.active_view().selected.0;
+        assert_eq!(last, 12, "should be at last row");
+        // With only ~5 visible rows and 13 rows, offset should scroll.
+        // Bug: hardcoded 20 means `12 >= 0 + 20` is false → no scroll.
+        let offset = app.model.active_view().row_offset;
+        assert!(
+            offset > 0,
+            "row_offset should scroll when last row is beyond visible area, but is {offset}"
+        );
+    }
+
+    #[test]
+    fn ctrl_d_scrolls_viewport_with_small_terminal() {
+        let mut app = two_col_model();
+        for i in 0..30 {
+            app.model
+                .category_mut("Row")
+                .unwrap()
+                .add_item(&format!("R{i}"));
+        }
+        app.term_height = 13; // ~5 visible rows
+        app.model.active_view_mut().selected = (0, 0);
+        // Ctrl+d scrolls by 5 rows
+        app.handle_key(KeyEvent::new(KeyCode::Char('d'), KeyModifiers::CONTROL))
+            .unwrap();
+        assert_eq!(app.model.active_view().selected.0, 5);
+        // Press Ctrl+d again — now at row 10 with only 5 visible rows,
+        // row_offset should have scrolled (not stay at 0 due to hardcoded 20)
+        app.handle_key(KeyEvent::new(KeyCode::Char('d'), KeyModifiers::CONTROL))
+            .unwrap();
+        assert_eq!(app.model.active_view().selected.0, 10);
+        assert!(
+            app.model.active_view().row_offset > 0,
+            "row_offset should scroll with small terminal, but is {}",
+            app.model.active_view().row_offset
+        );
+    }
+
     #[test]
     fn tab_in_edit_mode_commits_and_moves_right() {
         let mut app = two_col_model();