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

Add an AI client module that integrates with OpenAI-compatible API
providers (OpenAI, Perplexity, and custom endpoints) to provide
AI-assisted chat within CProof. Users can start sessions with /ai start,
send prompts, receive responses in a dedicated AI window, switch between
providers and models, and manage API keys — all with tab-completion.

Providers are configured via /ai set commands with per-provider API keys,
endpoints, default models, and custom settings. Two default providers
(openai, perplexity) are seeded on first use. Provider state persists in
[ai/<name>] sections of the preferences keyfile with automatic migration
from the previous flat-key format.

The /ai command integrates into the existing command system with 8
subcommands covering provider management, session lifecycle, model
fetching, and conversation clearing. Autocomplete uses the standard
flat prefix-matching chain for reliable tab-completion at every nesting
level. A new ProfAiWin window type is added to the window system.

Architecture:

Async design: HTTP requests run on a background thread (pthread) to avoid blocking the ncurses UI loop; results are displayed on the main thread via direct function calls
Thread safety: AIProvider and AISession use atomic ref-counting and mutex-protected session state; the request thread snapshots all session data before making the HTTP call
Window validation: wins_ai_exists() prevents use-after-free when the user closes the AI window during an in-flight HTTP request (~60s)
Privacy: store:false is sent with every request to prevent providers from persisting conversations or using them for training
Response size limit: 10MB cap with immediate curl abort via CURL_WRITEFUNC_ERROR to prevent OOM
JSON parsing uses unified helpers for both chat responses and error
envelopes with consistent escape decoding. The response parser tries
Perplexity /v1/responses "text" field first, then falls back to OpenAI
"content". Error parsing extracts provider error.message from the
standard envelope format. Model parsing handles multiple API response
formats (OpenAI list, Perplexity, array) including edge cases.

Tests include 470+ lines of unit tests covering provider management,
session lifecycle, JSON parsing (multiple formats), autocomplete cycling,
and error handling, plus functional tests for /ai command dispatch.
A stub_ai.c module isolates unit tests from UI dependencies.

src/command/cmd_ac.c

@@ -66,6 +66,8 @@
 #include "omemo/omemo.h"
 #endif
 
+#include "ai/ai_client.h"
+
 static char* _sub_autocomplete(ProfWin* window, const char* const input, gboolean previous);
 static char* _notify_autocomplete(ProfWin* window, const char* const input, gboolean previous);
 static char* _theme_autocomplete(ProfWin* window, const char* const input, gboolean previous);
@@ -137,6 +139,7 @@ static char* _strophe_autocomplete(ProfWin* window, const char* const input, gbo
 static char* _adhoc_cmd_autocomplete(ProfWin* window, const char* const input, gboolean previous);
 static char* _vcard_autocomplete(ProfWin* window, const char* const input, gboolean previous);
 static char* _force_encryption_autocomplete(ProfWin* window, const char* const input, gboolean previous);
+static char* _ai_autocomplete(ProfWin* window, const char* const input, gboolean previous);
 
 static char* _script_autocomplete_func(const char* const prefix, gboolean previous, void* context);
 
@@ -280,11 +283,16 @@ static Autocomplete history_switch_ac;
 static Autocomplete color_ac;
 static Autocomplete correction_ac;
 static Autocomplete avatar_ac;
+static Autocomplete ai_subcommands_ac;
+static Autocomplete ai_set_subcommands_ac;
+static Autocomplete ai_set_custom_subcommands_ac;
+static Autocomplete ai_remove_subcommands_ac;
 static Autocomplete url_ac;
 static Autocomplete executable_ac;
 static Autocomplete executable_param_ac;
 static Autocomplete intype_ac;
 static Autocomplete mood_ac;
+static Autocomplete ai_models_ac;
 static Autocomplete mood_type_ac;
 static Autocomplete strophe_ac;
 static Autocomplete strophe_sm_ac;
@@ -456,7 +464,12 @@ static Autocomplete* all_acs[] = {
     &vcard_togglable_param_ac,
     &vcard_address_type_ac,
     &force_encryption_ac,
-    &force_encryption_policy_ac
+    &force_encryption_policy_ac,
+    &ai_subcommands_ac,
+    &ai_set_subcommands_ac,
+    &ai_set_custom_subcommands_ac,
+    &ai_remove_subcommands_ac,
+    &ai_models_ac
 };
 
 static GHashTable* ac_funcs = NULL;
@@ -1169,6 +1182,26 @@ cmd_ac_init(void)
     autocomplete_add(correction_ac, "off");
     autocomplete_add(correction_ac, "char");
 
+    autocomplete_add_unsorted(ai_subcommands_ac, "set", FALSE);
+    autocomplete_add_unsorted(ai_subcommands_ac, "remove", FALSE);
+    autocomplete_add_unsorted(ai_subcommands_ac, "start", FALSE);
+    autocomplete_add_unsorted(ai_subcommands_ac, "clear", FALSE);
+    autocomplete_add_unsorted(ai_subcommands_ac, "providers", FALSE);
+    autocomplete_add_unsorted(ai_subcommands_ac, "switch", FALSE);
+    autocomplete_add_unsorted(ai_subcommands_ac, "models", FALSE);
+
+    autocomplete_add_unsorted(ai_set_subcommands_ac, "provider", FALSE);
+    autocomplete_add_unsorted(ai_set_subcommands_ac, "token", FALSE);
+    autocomplete_add_unsorted(ai_set_subcommands_ac, "default-model", FALSE);
+    autocomplete_add_unsorted(ai_set_subcommands_ac, "custom", FALSE);
+
+    autocomplete_add_unsorted(ai_set_custom_subcommands_ac, "tools", FALSE);
+    autocomplete_add_unsorted(ai_set_custom_subcommands_ac, "search", FALSE);
+    autocomplete_add_unsorted(ai_set_custom_subcommands_ac, "memory", FALSE);
+    autocomplete_add_unsorted(ai_set_custom_subcommands_ac, "plugins", FALSE);
+
+    autocomplete_add_unsorted(ai_remove_subcommands_ac, "provider", FALSE);
+
     autocomplete_add(avatar_ac, "set");
     autocomplete_add(avatar_ac, "disable");
     autocomplete_add(avatar_ac, "get");
@@ -1444,6 +1477,7 @@ cmd_ac_init(void)
     g_hash_table_insert(ac_funcs, "/wins", _wins_autocomplete);
     g_hash_table_insert(ac_funcs, "/wintitle", _wintitle_autocomplete);
     g_hash_table_insert(ac_funcs, "/force-encryption", _force_encryption_autocomplete);
+    g_hash_table_insert(ac_funcs, "/ai", _ai_autocomplete);
 }
 
 void
@@ -1662,6 +1696,7 @@ cmd_ac_reset(ProfWin* window)
     win_reset_search_attempts();
     win_close_reset_search_attempts();
     plugins_reset_autocomplete();
+    ai_providers_reset_ac();
 }
 
 void
@@ -4450,4 +4485,163 @@ _force_encryption_autocomplete(ProfWin* window, const char* const input, gboolea
 
     result = autocomplete_param_with_ac(input, "/force-encryption", force_encryption_ac, TRUE, previous);
     return result;
-}
+}
+
+/* Forward declaration */
+static char* _ai_provider_and_model_autocomplete(ProfWin* window, const char* const input,
+                                                 gboolean previous, const char* cmd);
+
+static char*
+_ai_autocomplete(ProfWin* window, const char* const input, gboolean previous)
+{
+    char* result = NULL;
+
+    /* Parse once for reuse - /ai <subcommand> [<arg1>] [<arg2>] */
+    gboolean parse_result = FALSE;
+    auto_gcharv gchar** args = parse_args(input, 1, 4, &parse_result);
+    int num_args = g_strv_length(args);
+
+    /* Top-level /ai <subcommand> autocomplete (e.g., /ai s<tab> -> /ai set) */
+    result = autocomplete_param_with_func(input, "/ai set provider", ai_providers_find, previous, NULL);
+    if (result) {
+        return result;
+    }
+
+    // /ai set token <provider> <token> - autocomplete provider names
+    result = autocomplete_param_with_func(input, "/ai set token", ai_providers_find, previous, NULL);
+    if (result) {
+        return result;
+    }
+
+    // /ai set default-model <provider> <model> - autocomplete provider names
+    result = autocomplete_param_with_func(input, "/ai set default-model", ai_providers_find, previous, NULL);
+    if (result) {
+        return result;
+    }
+
+    // /ai set custom <provider> <setting> <value> - autocomplete provider names
+    result = autocomplete_param_with_func(input, "/ai set custom", ai_providers_find, previous, NULL);
+    if (result) {
+        return result;
+    }
+
+    // /ai set custom <provider> <setting> - autocomplete settings
+    /* args[0]="set", args[1]="custom", args[2]=provider (if typed), args[3]=setting (if typed) */
+    if (num_args == 3 && g_strcmp0(args[1], "custom") == 0) {
+        /* /ai set custom <provider> - check if provider is valid */
+        if (ai_get_provider(args[2])) {
+            /* Valid provider, try settings autocomplete */
+            result = autocomplete_param_with_ac(input, "/ai set custom ", ai_set_custom_subcommands_ac, TRUE, previous);
+            if (result) {
+                return result;
+            }
+        }
+    }
+    // /ai set <subcommand> - autocomplete subcommands
+    result = autocomplete_param_with_ac(input, "/ai set", ai_set_subcommands_ac, TRUE, previous);
+    if (result) {
+        return result;
+    }
+
+    // /ai remove provider <name> - autocomplete provider names
+    result = autocomplete_param_with_func(input, "/ai remove provider", ai_providers_find, previous, NULL);
+    if (result) {
+        return result;
+    }
+
+    result = autocomplete_param_with_ac(input, "/ai remove", ai_remove_subcommands_ac, TRUE, previous);
+    if (result) {
+        return result;
+    }
+
+    // /ai start <provider> <model> - autocomplete provider names and model names
+    result = _ai_provider_and_model_autocomplete(window, input, previous, "/ai start");
+    if (result) {
+        return result;
+    }
+
+    // /ai switch <provider> <model> - autocomplete provider names and model names
+    result = _ai_provider_and_model_autocomplete(window, input, previous, "/ai switch");
+    if (result) {
+        return result;
+    }
+
+    // /ai set default-model <provider> <model> - autocomplete provider names and model names
+    result = _ai_provider_and_model_autocomplete(window, input, previous, "/ai set default-model");
+    if (result) {
+        return result;
+    }
+
+    // /ai models <provider> - autocomplete provider names
+    result = autocomplete_param_with_func(input, "/ai models", ai_providers_find, previous, NULL);
+    if (result) {
+        return result;
+    }
+
+    // /ai clear - no autocomplete
+    // /ai providers - no autocomplete
+
+    result = autocomplete_param_with_ac(input, "/ai", ai_subcommands_ac, FALSE, previous);
+
+    return result;
+}
+
+/**
+ * Autocomplete provider names and model names for /ai <cmd> <provider> <model> patterns.
+ * First tries to autocomplete provider names, then model names if provider is valid.
+ * Uses static ai_models_ac to preserve cycling state (last_found) across calls.
+ * The cmd_prefix should NOT include trailing space (e.g., "/ai start" not "/ai start ").
+ */
+static char*
+_ai_provider_and_model_autocomplete(ProfWin* window, const char* const input, gboolean previous, const char* cmd)
+{
+    char* result;
+
+    /* First, try provider name autocomplete */
+    result = autocomplete_param_with_func(input, cmd, ai_providers_find, previous, NULL);
+    if (result) {
+        return result;
+    }
+
+    /* Provider was specified, try model name autocomplete */
+    /* Build the full command prefix with space */
+    auto_gchar gchar* cmd_prefix = g_strdup_printf("%s ", cmd);
+
+    if (!g_str_has_prefix(input, cmd_prefix)) {
+        return NULL;
+    }
+
+    /* Extract provider name from input (after cmd_prefix) */
+    auto_gchar gchar* rest = g_strdup(input + strlen(cmd_prefix));
+    char* space = strchr(rest, ' ');
+    if (space) {
+        *space = '\0';
+    }
+
+    /* Look up the provider by name and get its models */
+    AIProvider* prov = ai_get_provider(rest);
+    if (!prov || !prov->models) {
+        return NULL;
+    }
+
+    if (!ai_models_ac) {
+        ai_models_ac = autocomplete_new();
+    }
+    /* Convert GList* to char** for autocomplete_update */
+    int model_count = g_list_length(prov->models);
+    char** model_array = g_new0(char*, model_count + 1);
+    GList* curr = prov->models;
+    int i = 0;
+    while (curr) {
+        model_array[i++] = curr->data;
+        curr = g_list_next(curr);
+    }
+
+    autocomplete_update(ai_models_ac, model_array);
+    g_free(model_array);
+
+    auto_gchar gchar* full_prefix = g_strdup_printf("%s%s", cmd_prefix, rest);
+
+    result = autocomplete_param_with_ac(input, full_prefix, ai_models_ac, TRUE, previous);
+    return result;
+}