refactor(engine): audit and rationalize platform cfg gates (closes #49)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

solitaire_engine/src/core_game_plugin.rs

@@ -8,7 +8,10 @@ use std::sync::Mutex;
 
 use bevy::prelude::*;
 
-use crate::platform::{StorageBackendResource, default_storage_backend};
+use crate::platform::{
+    ClipboardBackendResource, StorageBackendResource, default_clipboard_backend,
+    default_storage_backend,
+};
 use crate::{
     AchievementPlugin, AnalyticsPlugin, AnimationPlugin, AssetSourcesPlugin, AudioPlugin,
     AutoCompletePlugin, AvatarPlugin, CardAnimationPlugin, CardPlugin, ChallengePlugin,
@@ -53,6 +56,14 @@ impl Plugin for CoreGamePlugin {
                 warn!("storage: failed to initialize platform backend: {err}");
             }
         }
+        match default_clipboard_backend() {
+            Ok(clipboard) => {
+                app.insert_resource(ClipboardBackendResource(clipboard));
+            }
+            Err(err) => {
+                warn!("clipboard: failed to initialize platform backend: {err}");
+            }
+        }
 
         app.add_plugins(AssetSourcesPlugin)
             .add_plugins(ThemePlugin)