feat: expose ui_context_mut for engine-driven dropdown animation

The engine ticks the exposed context each loop — this drives the
dropdown expand/contract animation frames (the grown-trigger popover
treatment from cce-ui).

Co-Authored-By: Claude Fable 5 <[email protected]>

 src/main.rs | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/main.rs b/src/main.rs
index 6f1710a..fdbd79a 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -422,6 +422,12 @@ impl cce_ui::engine::Application for SystemInterface {
         Some(&self.ui_context)
     }
 
+    // The engine ticks the exposed context each loop — this is what drives the
+    // dropdown expand/contract animation frames.
+    fn ui_context_mut(&mut self) -> Option<&mut cce_ui::context::UiContext> {
+        Some(&mut self.ui_context)
+    }
+
     fn clear_color(&self) -> [f32; 4] {
         [0.039, 0.102, 0.055, 1.0]
     }