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/config/preferences.c

@@ -66,6 +66,7 @@
 #define PREF_GROUP_MUC           "muc"
 #define PREF_GROUP_PLUGINS       "plugins"
 #define PREF_GROUP_EXECUTABLES   "executables"
+#define PREF_GROUP_AI            "ai"
 
 #define INPBLOCK_DEFAULT 1000
 
@@ -1706,6 +1707,83 @@ _save_prefs(void)
     save_keyfile(&prefs_prof_keyfile);
 }
 
+/* ========================================================================
+ * AI Token Management
+ * ======================================================================== */
+
+gboolean
+prefs_ai_set_token(const char* const provider, const char* const token)
+{
+    if (!provider)
+        return FALSE;
+
+    if (!prefs)
+        return FALSE;
+
+    if (token) {
+        g_key_file_set_string(prefs, PREF_GROUP_AI, provider, token);
+        return TRUE;
+    } else {
+        return prefs_ai_remove_token(provider);
+    }
+}
+
+gboolean
+prefs_ai_remove_token(const char* const provider)
+{
+    if (!provider)
+        return FALSE;
+
+    if (!prefs)
+        return FALSE;
+
+    if (!g_key_file_has_key(prefs, PREF_GROUP_AI, provider, NULL)) {
+        return FALSE;
+    }
+
+    g_key_file_remove_key(prefs, PREF_GROUP_AI, provider, NULL);
+    return TRUE;
+}
+
+char*
+prefs_ai_get_token(const char* const provider)
+{
+    if (!provider || !prefs)
+        return NULL;
+
+    return g_key_file_get_string(prefs, PREF_GROUP_AI, provider, NULL);
+}
+
+GList*
+prefs_ai_list_tokens(void)
+{
+    if (!prefs || !g_key_file_has_group(prefs, PREF_GROUP_AI)) {
+        return NULL;
+    }
+
+    GList* result = NULL;
+    gsize len;
+    auto_gcharv gchar** keys = g_key_file_get_keys(prefs, PREF_GROUP_AI, &len, NULL);
+
+    for (gsize i = 0; i < len; i++) {
+        char* name = keys[i];
+        auto_gchar gchar* value = g_key_file_get_string(prefs, PREF_GROUP_AI, name, NULL);
+        if (value) {
+            result = g_list_append(result, g_strdup(name));
+        }
+    }
+
+    return result;
+}
+
+void
+prefs_free_ai_tokens(GList* tokens)
+{
+    if (tokens) {
+        g_list_free_full(tokens, g_free);
+    }
+}
+
 // get the preference group for a specific preference
 // for example the PREF_BEEP setting ("beep" in .profrc, see _get_key) belongs
 // to the [ui] section.
@@ -1865,6 +1943,9 @@ _get_group(preference_t pref)
         return PREF_GROUP_OMEMO;
     case PREF_OX_LOG:
         return PREF_GROUP_OX;
+    case PREF_AI_PROVIDER:
+    case PREF_AI_API_KEY:
+        return PREF_GROUP_AI;
     default:
         return NULL;
     }
@@ -2142,6 +2223,10 @@ _get_key(preference_t pref)
         return "stamp.incoming";
     case PREF_OX_LOG:
         return "log";
+    case PREF_AI_PROVIDER:
+        return "provider";
+    case PREF_AI_API_KEY:
+        return "api_key";
     case PREF_MOOD:
         return "mood";
     case PREF_VCARD_PHOTO_CMD:
@@ -2319,7 +2404,11 @@ _get_default_string(preference_t pref)
         return "on";
     case PREF_FORCE_ENCRYPTION_MODE:
         return "resend-to-confirm";
+    case PREF_AI_PROVIDER:
+        return "openai";
+    case PREF_AI_API_KEY:
+        return "";
     default:
         return NULL;
     }
-}
+}

src/config/preferences.h

@@ -183,6 +183,9 @@ typedef enum {
     PREF_OX_LOG,
     PREF_MOOD,
     PREF_STROPHE_VERBOSITY,
+    PREF_AI_PROVIDER,
+    PREF_AI_API_KEY,
+    PREF_AI_DEFAULT_MODEL,
     PREF_STROPHE_SM_ENABLED,
     PREF_STROPHE_SM_RESEND,
     PREF_VCARD_PHOTO_CMD,
@@ -356,4 +359,11 @@ gboolean prefs_get_room_notify(const char* const roomjid);
 gboolean prefs_get_room_notify_mention(const char* const roomjid);
 gboolean prefs_get_room_notify_trigger(const char* const roomjid);
 
+/* AI token management */
+gboolean prefs_ai_set_token(const char* const provider, const char* const token);
+gboolean prefs_ai_remove_token(const char* const provider);
+char* prefs_ai_get_token(const char* const provider);
+GList* prefs_ai_list_tokens(void);
+void prefs_free_ai_tokens(GList* tokens);
+
 #endif