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/events.rs

@@ -80,3 +80,10 @@ pub struct NewGameConfirmEvent;
 /// a short string to the player, e.g. "Locked — reach level 5".
 #[derive(Event, Debug, Clone)]
 pub struct InfoToastEvent(pub String);
+
+/// Fired by `ProgressPlugin` immediately after awarding XP for a win so the
+/// animation layer can display a "+N XP" toast alongside the win cascade.
+#[derive(Event, Debug, Clone, Copy)]
+pub struct XpAwardedEvent {
+    pub amount: u64,
+}