refactor(ai): remove redundant callback layer from AI client

The ai_response_cb and ai_error_cb parameters were always passed the
same functions (aiwin_display_response and aiwin_display_error),
making the callback indirection redundant and complicating the call
chain.

Remove ai_callback_data_t, _ai_callback_invoke(), and
_ai_invoke_callback(). Simplify _ai_request_thread and ai_send_prompt
to directly call aiwin_display_error() and aiwin_display_response()
when user_data is a valid ProfAiWin*.

src/ai/ai_client.h

@@ -3,20 +3,6 @@
 
 #include <glib.h>
 
-/**
- * @brief Callback function for successful AI responses.
- * @param response The AI response text.
- * @param user_data User-provided data passed to the callback.
- */
-typedef void (*ai_response_cb)(const gchar* response, gpointer user_data);
-
-/**
- * @brief Callback function for AI errors.
- * @param error_msg The error message.
- * @param user_data User-provided data passed to the callback.
- */
-typedef void (*ai_error_cb)(const gchar* error_msg, gpointer user_data);
-
 /**
  * @brief AI message structure for conversation history.
  */
@@ -186,7 +172,7 @@ void ai_session_set_model(AISession* session, const gchar* model);
  * Send a prompt to the AI provider asynchronously.
  * @param session The AI session containing provider and model
  * @param prompt The prompt to send
- * @param user_data User data to be passed to the callbacks
+ * @param user_data User data (ProfAiWin* for UI display)
  * @return TRUE if the request was successfully queued, FALSE otherwise
  */
 gboolean ai_send_prompt(AISession* session, const gchar* prompt,