feat(engine,server): XP toast on win + display_name max-length validation

ProgressPlugin now fires XpAwardedEvent on every win. AnimationPlugin
shows a "+N XP" toast so players see XP feedback immediately after
winning. Server leaderboard opt-in endpoint also now validates that
display_name is at most 32 characters.

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

solitaire_engine/src/lib.rs

@@ -41,7 +41,7 @@ pub use card_plugin::{CardEntity, CardLabel, CardPlugin};
 pub use events::{
     AchievementUnlockedEvent, CardFlippedEvent, DrawRequestEvent, GameWonEvent, InfoToastEvent,
     ManualSyncRequestEvent, MoveRejectedEvent, MoveRequestEvent, NewGameConfirmEvent,
-    NewGameRequestEvent, StateChangedEvent, UndoRequestEvent,
+    NewGameRequestEvent, StateChangedEvent, UndoRequestEvent, XpAwardedEvent,
 };
 pub use game_plugin::{GameMutation, GamePlugin, GameStatePath};
 pub use help_plugin::{HelpPlugin, HelpScreen};