feat: handle StopFailure hook event for API error turns (#224)

Parses [StopFailure Hook] from stderr, emits claude:stop-failure, and
transitions the character to error state with a toast notification.

src/lib/stores/toasts.ts

@@ -68,6 +68,13 @@ function createToastStore() {
     setTimeout(() => remove(id), 4000);
   }
 
+  function addError(message: string) {
+    const id = crypto.randomUUID();
+    const toast: InfoToast = { id, kind: "info", message, icon: "⚠️" };
+    update((toasts) => [...toasts, toast]);
+    setTimeout(() => remove(id), 6000);
+  }
+
   function addAchievement(achievement: AchievementUnlockedEvent["achievement"]) {
     const id = crypto.randomUUID();
     const toast: AchievementToast = { id, kind: "achievement", achievement };
@@ -82,7 +89,7 @@ function createToastStore() {
     // Update toasts are persistent — no auto-dismiss
   }
 
-  return { subscribe, addInfo, addAchievement, addUpdate, remove };
+  return { subscribe, addInfo, addError, addAchievement, addUpdate, remove };
 }
 
 export const toastStore = createToastStore();