fix(app): wrap WinitWindows in Option to satisfy Bevy 0.18 param validation

`NonSend<WinitWindows>` failed system-param validation on the
first few frames before `WinitWindows` was populated, panicking
the Update system before any logic could run. Bevy 0.18's
stricter validation panics rather than skips when a non-send
resource is absent, with an error message spelling out the fix:
*"wrap the parameter in `Option<T>` and handle `None` when it
happens."*

Wraps `winit_windows` as `Option<NonSend<WinitWindows>>` and
early-returns on `None`, mirroring the same lifecycle handling
already applied to `winit_windows.get_window(primary_entity)` —
both fail in the same window of frames before winit's `Resumed`
event fires.

Repro from the user's `cargo run` log:
```
thread 'Compute Task Pool (2)' panicked at .../bevy_ecs-0.18.1/src/error/handler.rs:125:1:
Encountered an error in system ...: Parameter ... failed validation:
Non-send resource does not exist
```

Workspace clippy + cargo test --workspace clean, 1185 passing.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

solitaire_app/src/lib.rs

@@ -284,11 +284,20 @@ fn apply_smart_default_window_size(
 fn set_window_icon(
     mut applied: Local<bool>,
     primary_window: Query<Entity, With<PrimaryWindow>>,
-    winit_windows: NonSend<WinitWindows>,
+    // `Option<NonSend<...>>` rather than `NonSend<...>` because Bevy
+    // 0.18's stricter system-param validation panics on the first
+    // few frames before `WinitWindows` is inserted (the resource is
+    // populated after winit's `Resumed` event, which fires after
+    // the first system-tick batch). The early-return below handles
+    // the `None` window-wrapper case for the same lifecycle reason.
+    winit_windows: Option<NonSend<WinitWindows>>,
 ) {
     if *applied {
         return;
     }
+    let Some(winit_windows) = winit_windows else {
+        return;
+    };
     let Ok(primary_entity) = primary_window.single() else {
         return;
     };