fix(ai): validate aiwin pointer to prevent UAF when window closed

Prevent use-after-free in the AI request worker thread when the user
closes the AI window during the 60-second HTTP request timeout. Without
this fix, the worker may dereference a dangling pointer and crash or
exhibit undefined behavior.

The _ai_request_thread function in ai_client.c previously cast user_data
to ProfAiWin* and used it directly without verifying the window was still
alive. Added wins_ai_exists() to iterate through all AI windows and check
if the pointer is still valid before use.

Safety:
- wins_ai_exists() iterates all AI windows (handles multiple AI windows)
- Worker skips UI update if window was freed, just cleans up
- Logs warning when dangling pointer is detected

src/ui/window_list.h

@@ -64,6 +64,7 @@ ProfPluginWin* wins_get_plugin(const char* const tag);
 ProfXMLWin* wins_get_xmlconsole(void);
 ProfVcardWin* wins_get_vcard(void);
 ProfAiWin* wins_get_ai(void);
+gboolean wins_ai_exists(ProfAiWin* const aiwin);
 
 void wins_close_plugin(char* tag);