feat(ai): add AI client with multi-provider support and UI

Add an AI client module that integrates with OpenAI-compatible API
providers (OpenAI, Perplexity, and custom providers) to provide
AI-assisted responses within the profanity client.

The implementation includes:

- src/ai/ai_client.c/h: Core AI client with provider management,
  session handling, and async HTTP request handling via libcurl.
  Supports per-provider API keys stored in preferences, reference-
  counted sessions, and conversation history tracking.

- src/ui/window.c/window_list.c: New AI window type (ProfAiWin) for
  displaying AI conversations, with response streaming and error
  display capabilities.

- Command integration: New `/ai` command (cmd_defs.c, cmd_funcs.c)
  for creating sessions, sending prompts, and managing providers.
  Provider autocomplete support in cmd_ac.c.

- Preferences integration: API keys for providers are persisted in
  the preferences system (config/preferences.c).

- Unit tests: 472 lines of comprehensive tests covering provider
  management, session lifecycle, JSON escaping, and autocomplete
  (tests/unittests/test_ai_client.c).

Architecture decisions:
- Asynchronous design: HTTP requests run on a separate thread to
  avoid blocking the main UI loop. Callbacks are invoked on the main
  thread via direct function call (profanity uses ncurses, not GLib
  main loop).
- Reference counting: Both AIProvider and AISession use ref counting
  for safe shared ownership.
- Response size limit: 10MB cap on HTTP responses to prevent OOM.

src/profanity.c

@@ -69,6 +69,7 @@
 #include "xmpp/xmpp.h"
 #include "xmpp/muc.h"
 #include "xmpp/chat_session.h"
+#include "ai/ai_client.h"
 #include "xmpp/chat_state.h"
 #include "xmpp/contact.h"
 #include "xmpp/roster_list.h"
@@ -249,6 +250,7 @@ _init(char* log_level, char* config_file, char* log_file, char* theme_name)
     }
     session_init();
     cmd_init();
+    ai_client_init();
     log_info("Initialising contact list");
     muc_init();
     tlscerts_init();