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.

tests/unittests/ui/stub_ai.c

@@ -0,0 +1,30 @@
+/*
+ * stub_ai.c - Stubs for AI window functions
+ */
+
+#include "ui/window.h"
+#include "ai/ai_client.h"
+
+ProfWin*
+win_create_ai(AISession* session)
+{
+    /* Return NULL to simulate failure in unit tests */
+    (void)session;
+    return NULL;
+}
+
+void
+aiwin_display_response(ProfAiWin* win, const char* response)
+{
+    /* Stub: do nothing */
+    (void)win;
+    (void)response;
+}
+
+void
+aiwin_display_error(ProfAiWin* win, const char* error_msg)
+{
+    /* Stub: do nothing */
+    (void)win;
+    (void)error_msg;
+}