feat(engine): fetch daily-challenge seed from server on startup

- Add fetch_daily_challenge() to SyncProvider trait (default: Ok(None))
- SolitaireServerClient calls GET /api/daily-challenge (public endpoint)
  and returns the ChallengeGoal; non-2xx responses return Ok(None) so
  callers fall back to the local date-hash seed
- DailyChallengePlugin spawns an async task on Startup (only when
  SyncProviderResource is present) and polls it in Update; on success
  it overwrites DailyChallengeResource.seed with the server's seed,
  ensuring all players worldwide get the same deal on a given date

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

solitaire_data/src/lib.rs

@@ -1,5 +1,5 @@
 use async_trait::async_trait;
-use solitaire_sync::{LeaderboardEntry, SyncPayload, SyncResponse};
+use solitaire_sync::{ChallengeGoal, LeaderboardEntry, SyncPayload, SyncResponse};
 use thiserror::Error;
 
 /// All errors that can arise during sync operations.
@@ -36,6 +36,11 @@ pub trait SyncProvider: Send + Sync {
     async fn fetch_leaderboard(&self) -> Result<Vec<LeaderboardEntry>, SyncError> {
         Ok(vec![])
     }
+    /// Fetch today's daily challenge from the server. Returns `None` for
+    /// backends that don't support it, or on any non-fatal network failure.
+    async fn fetch_daily_challenge(&self) -> Result<Option<ChallengeGoal>, SyncError> {
+        Ok(None)
+    }
 }
 
 /// Blanket impl so `Box<dyn SyncProvider + Send + Sync>` (returned by
@@ -60,6 +65,9 @@ impl SyncProvider for Box<dyn SyncProvider + Send + Sync> {
     async fn fetch_leaderboard(&self) -> Result<Vec<LeaderboardEntry>, SyncError> {
         (**self).fetch_leaderboard().await
     }
+    async fn fetch_daily_challenge(&self) -> Result<Option<ChallengeGoal>, SyncError> {
+        (**self).fetch_daily_challenge().await
+    }
 }
 
 pub mod stats;