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.c

@@ -36,6 +36,7 @@
 
 #include "config.h"
 #include "database.h"
+#include "ui/win_types.h"
 #include "ui/window_list.h"
 
 #include <stdlib.h>
@@ -62,6 +63,7 @@
 #include "ui/screen.h"
 #include "xmpp/xmpp.h"
 #include "xmpp/roster_list.h"
+#include "ai/ai_client.h"
 
 static const char* LOADING_MESSAGE = "Loading older messages…";
 static const char* CONS_WIN_TITLE = "CProof. Type /help for help information.";
@@ -329,6 +331,33 @@ win_create_vcard(vCard* vcard)
     return &new_win->window;
 }
 
+ProfWin*
+win_create_ai(AISession* session)
+{
+    ProfAiWin* new_win = g_new0(ProfAiWin, 1);
+    new_win->window.type = WIN_AI;
+    new_win->window.scroll_state = WIN_SCROLL_INNER;
+    new_win->window.layout = _win_create_simple_layout();
+
+    new_win->message_buffer = buffer_create();
+    new_win->conversation_history = g_string_new("");
+    new_win->session = session ? ai_session_ref(session) : NULL;
+    new_win->memcheck = PROFAIWIN_MEMCHECK;
+
+    return &new_win->window;
+}
+
+void
+win_ai_free(ProfAiWin* win)
+{
+    assert(win->memcheck == PROFAIWIN_MEMCHECK);
+    buffer_free(win->message_buffer);
+    g_string_free(win->conversation_history, TRUE);
+    if (win->session) {
+        ai_session_unref(win->session);
+    }
+}
+
 gchar*
 win_get_title(ProfWin* window)
 {
@@ -407,6 +436,13 @@ win_get_title(ProfWin* window)
 
         return g_string_free(title, FALSE);
     }
+    case WIN_AI:
+    {
+        const ProfAiWin* aiwin = (ProfAiWin*)window;
+        assert(aiwin->memcheck == PROFAIWIN_MEMCHECK);
+
+        return g_strdup_printf("AI Assistant (%s: %s)", aiwin->session->provider_name, aiwin->session->model);
+    }
     }
     assert(FALSE);
 }
@@ -454,6 +490,10 @@ win_get_tab_identifier(ProfWin* window)
     {
         return strdup("vcard");
     }
+    case WIN_AI:
+    {
+        return strdup("ai");
+    }
     }
     assert(FALSE);
 }
@@ -536,6 +576,12 @@ win_to_string(ProfWin* window)
         ProfVcardWin* vcardwin = (ProfVcardWin*)window;
         return vcardwin_get_string(vcardwin);
     }
+    case WIN_AI:
+    {
+        ProfAiWin* aiwin = (ProfAiWin*)window;
+        assert(aiwin->memcheck == PROFAIWIN_MEMCHECK);
+        return aiwin_get_string(aiwin);
+    }
     }
     assert(FALSE);
 }
@@ -662,6 +708,17 @@ win_free(ProfWin* window)
         free(pluginwin->plugin_name);
         break;
     }
+    case WIN_AI:
+    {
+        ProfAiWin* aiwin = (ProfAiWin*)window;
+        assert(aiwin->memcheck == PROFAIWIN_MEMCHECK);
+        buffer_free(aiwin->message_buffer);
+        g_string_free(aiwin->conversation_history, TRUE);
+        if (aiwin->session) {
+            ai_session_unref(aiwin->session);
+        }
+        break;
+    }
     default:
         break;
     }
@@ -2367,3 +2424,69 @@ get_enc_char(prof_enc_t enc_mode, const char* alt)
     else
         return get_show_char(enc_mode);
 }
+
+// AI Window Functions
+
+char*
+aiwin_get_string(ProfAiWin* win)
+{
+    assert(win->memcheck == PROFAIWIN_MEMCHECK);
+    return g_strdup_printf("AI Chat (%s: %s)", win->session->provider_name, win->session->model);
+}
+
+void
+aiwin_send_message(ProfAiWin* win, const char* message, const char* id)
+{
+    log_debug("[AI-WIN] aiwin_send_message ENTER: win=%p, message='%s'", (void*)win, message);
+    assert(win->memcheck == PROFAIWIN_MEMCHECK);
+
+    if (!win->session) {
+        log_error("[AI-WIN] FAIL: AI session not initialized");
+        return;
+    }
+
+    // Display user message
+    win_print_outgoing(&win->window, ">>", id, NULL, message);
+    log_debug("[AI-WIN] Displayed user message");
+
+    // Send to AI (ai_send_prompt adds message to history internally)
+    log_debug("[AI-WIN] Calling ai_send_prompt...");
+    gboolean result = ai_send_prompt(win->session, message,
+                                     (ai_response_cb)aiwin_display_response,
+                                     (ai_error_cb)aiwin_display_error,
+                                     win);
+    log_debug("[AI-WIN] ai_send_prompt returned: %d", result);
+}
+
+void
+aiwin_display_response(ProfAiWin* win, const char* response)
+{
+    log_debug("[AI-WIN] aiwin_display_response ENTER: win=%p, response='%s'", (void*)win, response);
+    assert(win->memcheck == PROFAIWIN_MEMCHECK);
+
+    if (!response) {
+        log_error("[AI-WIN] FAIL: null response");
+        return;
+    }
+
+    // Display AI response
+    win_println(&win->window, THEME_DEFAULT, "<< LLM:", "%s", response);
+    log_debug("[AI-WIN] Displayed AI response");
+
+    // Add to conversation history
+    ai_session_add_message(win->session, "assistant", response);
+    log_debug("[AI-WIN] Added assistant message to session history");
+}
+
+void
+aiwin_display_error(ProfAiWin* win, const char* error_msg)
+{
+    log_debug("[AI-WIN] aiwin_display_error ENTER: win=%p, error='%s'", (void*)win, error_msg);
+    assert(win->memcheck == PROFAIWIN_MEMCHECK);
+
+    const char* msg = error_msg ? error_msg : "Unknown error";
+    win_println(&win->window, THEME_ERROR, "[ERROR]", "%s", msg);
+
+    // Also show error in console so user sees it regardless of active window
+    cons_show_error("AI Error: %s", msg);
+}