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/ui/window.h

@@ -101,4 +101,13 @@ char* win_quote_autocomplete(ProfWin* window, const char* const input, gboolean
 char* get_show_char(prof_enc_t encryption_mode);
 char* get_enc_char(prof_enc_t enc_mode, const char* alt);
 
+#include "ai/ai_client.h"
+
+// AI window functions
+void win_ai_free(ProfAiWin* win);
+char* aiwin_get_string(ProfAiWin* win);
+void aiwin_send_message(ProfAiWin* win, const char* message, const char* id);
+void aiwin_display_response(ProfAiWin* win, const char* response);
+void aiwin_display_error(ProfAiWin* win, const char* error_msg);
+
 #endif