fix(android): add type annotation for hotkey None in spawn_modal_button

The Android aarch64 compiler cannot infer the type of `let hotkey = None` inside
the `#[cfg(target_os = "android")]` block — it needs to know the Option's inner
type to resolve the rebinding downstream. Added `: Option<&'static str>` to match
the parameter type and match the non-Android path.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/src/ui_modal.rs

@@ -329,7 +329,7 @@ pub fn spawn_modal_button<M: Component>(
     font_res: Option<&FontResource>,
 ) {
     #[cfg(target_os = "android")]
-    let hotkey = None;
+    let hotkey: Option<&'static str> = None;
     let font_handle = font_res.map(|f| f.0.clone()).unwrap_or_default();
     let font_label = TextFont {
         font: font_handle.clone(),