feat(ui): add footer support to panels

Add footer_height method to PanelContent trait with default 0. Implement
footer_height in FormulaContent to return 1 when in FormulaEdit mode.
Update Panel::render to subtract footer height from item height. This
enables optional footers in panels.

Co-Authored-By: fiddlerwoaroof/git-smart-commit (bartowski/nvidia_Nemotron-Cascade-2-30B-A3B-GGUF)

src/ui/panel.rs

@@ -22,7 +22,11 @@ pub trait PanelContent {
     /// Render a single item at the given row index.
     /// `inner` is the full inner area of the panel; the item occupies row `index`.
     fn render_item(&self, index: usize, is_selected: bool, inner: Rect, buf: &mut Buffer);
-    /// Optional footer rendered below the items (e.g. inline hints).
+    /// Number of lines the footer occupies (used to reserve space).
+    fn footer_height(&self) -> u16 {
+        0
+    }
+    /// Optional footer rendered in the reserved space at the bottom.
     fn render_footer(&self, _inner: Rect, _buf: &mut Buffer) {}
 }
 
@@ -69,8 +73,9 @@ impl<C: PanelContent> Widget for Panel<'_, C> {
             return;
         }
 
+        let item_height = inner.height.saturating_sub(self.content.footer_height());
         for i in 0..self.content.item_count() {
-            if i as u16 >= inner.height {
+            if i as u16 >= item_height {
                 break;
             }
             let is_selected = i == self.cursor && is_active;