feat(workspace): initialize all seven crates with stubs and blank Bevy window

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

solitaire_gpgs/src/stub.rs

@@ -0,0 +1,38 @@
+use async_trait::async_trait;
+use solitaire_data::{SyncError, SyncProvider};
+use solitaire_sync::{SyncPayload, SyncResponse};
+
+/// Desktop/iOS stub — always returns UnsupportedPlatform.
+/// Real implementation lives in android.rs (Phase: Android).
+pub struct GpgsClient;
+
+impl GpgsClient {
+    pub fn new() -> Self {
+        Self
+    }
+}
+
+impl Default for GpgsClient {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+#[async_trait]
+impl SyncProvider for GpgsClient {
+    async fn pull(&self) -> Result<SyncPayload, SyncError> {
+        Err(SyncError::UnsupportedPlatform)
+    }
+
+    async fn push(&self, _payload: &SyncPayload) -> Result<SyncResponse, SyncError> {
+        Err(SyncError::UnsupportedPlatform)
+    }
+
+    fn backend_name(&self) -> &'static str {
+        "Google Play Games (unavailable on this platform)"
+    }
+
+    fn is_authenticated(&self) -> bool {
+        false
+    }
+}