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/event/client_events.c

@@ -37,8 +37,10 @@
 #include "config.h"
 
 #include <stdlib.h>
+#include <assert.h>
 #include <glib.h>
 
+#include "ai/ai_client.h"
 #include "log.h"
 #include "chatlog.h"
 #include "database.h"
@@ -47,7 +49,7 @@
 #include "event/common.h"
 #include "plugins/plugins.h"
 #include "ui/inputwin.h"
-#include "ui/window_list.h"
+#include "ui/window.h"
 #include "xmpp/chat_session.h"
 #include "xmpp/session.h"
 #include "xmpp/xmpp.h"
@@ -284,3 +286,30 @@ allow_unencrypted_message(ProfChatWin* chatwin, const char* const msg)
     win_println((ProfWin*)chatwin, THEME_ERROR, "-", "Message not sent: invalid encryption mode (%s). Use '/force-encryption policy resend-to-confirm'.", force_enc_mode);
     return FALSE;
 }
+
+void
+cl_ev_send_ai_msg(ProfAiWin* aiwin, const char* const message, const char* const id)
+{
+    if (!aiwin || !message) {
+        return;
+    }
+
+    assert(aiwin->memcheck == PROFAIWIN_MEMCHECK);
+
+    if (!aiwin->session) {
+        log_error("[AI] AI session not initialized");
+        return;
+    }
+
+    /* Reset paged flag before printing user message.
+     * If the user scrolled up to view history, paged=1 would suppress
+     * the message in _win_printf(). Reset it here so the message displays. */
+    aiwin->window.layout->paged = 0;
+    aiwin->window.layout->unread_msg = 0;
+
+    // Display user message in AI window.
+    win_print_outgoing(&aiwin->window, ">>", id, NULL, message);
+
+    // Send to AI provider.
+    ai_send_prompt(aiwin->session, message, aiwin);
+}

src/event/client_events.h

@@ -51,6 +51,7 @@ void cl_ev_send_msg(ProfChatWin* chatwin, const char* const msg, const char* con
 void cl_ev_send_muc_msg_corrected(ProfMucWin* mucwin, const char* const msg, const char* const oob_url, gboolean correct_last_msg);
 void cl_ev_send_muc_msg(ProfMucWin* mucwin, const char* const msg, const char* const oob_url);
 void cl_ev_send_priv_msg(ProfPrivateWin* privwin, const char* const msg, const char* const oob_url);
+void cl_ev_send_ai_msg(ProfAiWin* aiwin, const char* const message, const char* const id);
 
 // Checks if an unencrypted message can be sent based on encryption preferences.
 // Returns TRUE if allowed, FALSE if blocked.