feat: handle PostCompact hook event (#225)

Parses [PostCompact Hook] from stderr, emits claude:post-compact, shows
a toast notification, and briefly sets the character to success state.

src-tauri/src/types.rs

@@ -310,6 +310,14 @@ pub struct StopFailureEvent {
     pub conversation_id: Option<String>,
 }
 
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct PostCompactEvent {
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub session_id: 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,
@@ -697,4 +705,40 @@ mod tests {
         assert!(!serialized.contains("error_type"));
         assert!(!serialized.contains("conversation_id"));
     }
+
+    #[test]
+    fn test_post_compact_event_serialization() {
+        let event = PostCompactEvent {
+            session_id: Some("sess-abc".to_string()),
+            conversation_id: Some("conv-123".to_string()),
+        };
+
+        let serialized = serde_json::to_string(&event).unwrap();
+        assert!(serialized.contains("\"session_id\":\"sess-abc\""));
+        assert!(serialized.contains("\"conversation_id\":\"conv-123\""));
+    }
+
+    #[test]
+    fn test_post_compact_event_omits_none_fields() {
+        let event = PostCompactEvent {
+            session_id: None,
+            conversation_id: None,
+        };
+
+        let serialized = serde_json::to_string(&event).unwrap();
+        assert!(!serialized.contains("session_id"));
+        assert!(!serialized.contains("conversation_id"));
+    }
+
+    #[test]
+    fn test_post_compact_event_partial_fields() {
+        let event = PostCompactEvent {
+            session_id: Some("sess-xyz".to_string()),
+            conversation_id: None,
+        };
+
+        let serialized = serde_json::to_string(&event).unwrap();
+        assert!(serialized.contains("\"session_id\":\"sess-xyz\""));
+        assert!(!serialized.contains("conversation_id"));
+    }
 }