fix(workspace): add derives/docs per code review, remove unused thiserror from solitaire_sync

solitaire_data/src/lib.rs

@@ -7,6 +7,8 @@ use thiserror::Error;
 pub enum SyncError {
     #[error("unsupported platform for this sync backend")]
     UnsupportedPlatform,
+    // TODO: Replace String with concrete source error types (e.g. reqwest::Error,
+    // serde_json::Error) when real implementations are added in Phase 8.
     #[error("network error: {0}")]
     Network(String),
     #[error("authentication error: {0}")]
@@ -19,9 +21,13 @@ pub enum SyncError {
 /// methods — it never matches on a backend enum variant.
 #[async_trait]
 pub trait SyncProvider: Send + Sync {
+    /// Fetch the remote sync payload. Returns the latest server state for merging.
     async fn pull(&self) -> Result<SyncPayload, SyncError>;
+    /// Push the local payload to the backend. Returns the merged server response.
     async fn push(&self, payload: &SyncPayload) -> Result<SyncResponse, SyncError>;
+    /// Human-readable name of this backend, used in settings UI and logs.
     fn backend_name(&self) -> &'static str;
+    /// Returns true if the user is currently authenticated with this backend.
     fn is_authenticated(&self) -> bool;
     /// Mirror an achievement unlock to this backend (no-op for most backends).
     async fn mirror_achievement(&self, _id: &str) -> Result<(), SyncError> {