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-tauri/src/types.rs

@@ -300,6 +300,16 @@ pub struct ElicitationResultEvent {
     pub conversation_id: Option<String>,
 }
 
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct StopFailureEvent {
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub stop_reason: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub error_type: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub conversation_id: Option<String>,
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct AgentStartEvent {
     pub tool_use_id: String,
@@ -645,4 +655,46 @@ mod tests {
         assert!(!serialized.contains("request_id"));
         assert!(!serialized.contains("conversation_id"));
     }
+
+    #[test]
+    fn test_stop_failure_event_serialization() {
+        let event = StopFailureEvent {
+            stop_reason: Some("api_error".to_string()),
+            error_type: Some("rate_limit".to_string()),
+            conversation_id: Some("conv-abc".to_string()),
+        };
+
+        let serialized = serde_json::to_string(&event).unwrap();
+        assert!(serialized.contains("\"stop_reason\":\"api_error\""));
+        assert!(serialized.contains("\"error_type\":\"rate_limit\""));
+        assert!(serialized.contains("\"conversation_id\":\"conv-abc\""));
+    }
+
+    #[test]
+    fn test_stop_failure_event_omits_none_fields() {
+        let event = StopFailureEvent {
+            stop_reason: None,
+            error_type: None,
+            conversation_id: None,
+        };
+
+        let serialized = serde_json::to_string(&event).unwrap();
+        assert!(!serialized.contains("stop_reason"));
+        assert!(!serialized.contains("error_type"));
+        assert!(!serialized.contains("conversation_id"));
+    }
+
+    #[test]
+    fn test_stop_failure_event_partial_fields() {
+        let event = StopFailureEvent {
+            stop_reason: Some("api_error".to_string()),
+            error_type: None,
+            conversation_id: None,
+        };
+
+        let serialized = serde_json::to_string(&event).unwrap();
+        assert!(serialized.contains("\"stop_reason\":\"api_error\""));
+        assert!(!serialized.contains("error_type"));
+        assert!(!serialized.contains("conversation_id"));
+    }
 }