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/ai/ai_client.h

@@ -0,0 +1,204 @@
+#ifndef AI_CLIENT_H
+#define AI_CLIENT_H
+
+#include <glib.h>
+
+/**
+ * @brief Callback function for successful AI responses.
+ * @param response The AI response text.
+ * @param user_data User-provided data passed to the callback.
+ */
+typedef void (*ai_response_cb)(const gchar* response, gpointer user_data);
+
+/**
+ * @brief Callback function for AI errors.
+ * @param error_msg The error message.
+ * @param user_data User-provided data passed to the callback.
+ */
+typedef void (*ai_error_cb)(const gchar* error_msg, gpointer user_data);
+
+/**
+ * @brief AI message structure for conversation history.
+ */
+typedef struct ai_message_t
+{
+    gchar* role;    /* "user" or "assistant" */
+    gchar* content; /* Message content */
+} AIMessage;
+
+/**
+ * @brief AI provider configuration.
+ */
+typedef struct ai_provider_t
+{
+    gchar* name;       /* Provider name (e.g., "openai", "perplexity") */
+    gchar* api_url;    /* API endpoint URL */
+    gchar* org_id;     /* Optional organization ID */
+    gchar* project_id; /* Optional project ID (for some providers) */
+    GList* models;     /* List of available models (gchar*) */
+    guint ref_count;   /* Reference count */
+} AIProvider;
+
+/**
+ * @brief AI chat session structure.
+ */
+typedef struct ai_session_t
+{
+    gchar* provider_name; /* Provider name */
+    AIProvider* provider; /* Provider configuration */
+    gchar* model;         /* Model identifier (e.g., "gpt-4", "sonar") */
+    gchar* api_key;       /* API key for this session */
+    GList* history;       /* Conversation history (GList of AIMessage*) */
+    guint ref_count;      /* Reference count */
+} AISession;
+
+/* ========================================================================
+ * Provider Management
+ * ======================================================================== */
+
+/**
+ * Initialize the AI client and load default providers.
+ */
+void ai_client_init(void);
+
+/**
+ * Shutdown the AI client and free resources.
+ */
+void ai_client_shutdown(void);
+
+/**
+ * Get a provider by name.
+ * @param name The provider name (e.g., "openai", "perplexity")
+ * @return AIProvider*, or NULL if not found
+ */
+AIProvider* ai_get_provider(const gchar* name);
+
+/**
+ * Add or update a provider configuration.
+ * @param name The provider name
+ * @param api_url The API endpoint URL
+ * @param org_id Optional organization ID (can be NULL)
+ * @return New AIProvider* (caller must unref when done)
+ */
+AIProvider* ai_add_provider(const gchar* name, const gchar* api_url, const gchar* org_id);
+
+/**
+ * Remove a provider by name.
+ * @param name The provider name
+ * @return TRUE if provider was removed, FALSE if not found
+ */
+gboolean ai_remove_provider(const gchar* name);
+
+/**
+ * Decrement the reference count of a provider.
+ * @param provider The provider to unreference
+ */
+void ai_provider_unref(AIProvider* provider);
+
+/**
+ * List all configured providers.
+ * @return GList of AIProvider* (caller must not free the list or providers)
+ */
+GList* ai_list_providers(void);
+
+/**
+ * Find a provider name for autocomplete.
+ * @param search_str The search string
+ * @param previous Whether to go to previous match
+ * @param context Unused
+ * @return Provider name, or NULL if not found
+ */
+char* ai_providers_find(const char* const search_str, gboolean previous, void* context);
+
+/**
+ * Get the API key for a provider.
+ * @param provider_name The provider name
+ * @return The API key, or NULL if not set (caller must free)
+ */
+gchar* ai_get_provider_key(const gchar* provider_name);
+
+/**
+ * Set the API key for a provider.
+ * @param provider_name The provider name
+ * @param api_key The API key to set
+ */
+void ai_set_provider_key(const gchar* provider_name, const gchar* api_key);
+
+/* ========================================================================
+ * Session Management
+ * ======================================================================== */
+
+/**
+ * Create a new AI session with the specified provider and model.
+ * @param provider_name The provider name (e.g., "openai")
+ * @param model The model identifier (e.g., "gpt-4")
+ * @return New AISession*, or NULL on failure
+ */
+AISession* ai_session_create(const gchar* provider_name, const gchar* model);
+
+/**
+ * Increment the reference count of an AI session.
+ * @param session The session to reference
+ * @return The same session pointer
+ */
+AISession* ai_session_ref(AISession* session);
+
+/**
+ * Decrement the reference count and free the session when it reaches zero.
+ * @param session The session to unreference
+ */
+void ai_session_unref(AISession* session);
+
+/**
+ * Add a message to the session history.
+ * @param session The session
+ * @param role The message role ("user" or "assistant")
+ * @param content The message content
+ */
+void ai_session_add_message(AISession* session, const gchar* role, const gchar* content);
+
+/**
+ * Clear the conversation history.
+ * @param session The session
+ */
+void ai_session_clear_history(AISession* session);
+
+/**
+ * Get the current model for a session.
+ * @param session The session
+ * @return The model name (caller must not free)
+ */
+const gchar* ai_session_get_model(AISession* session);
+
+/**
+ * Set the model for a session.
+ * @param session The session
+ * @param model The model name
+ */
+void ai_session_set_model(AISession* session, const gchar* model);
+
+/* ========================================================================
+ * Request Handling
+ * ======================================================================== */
+
+/**
+ * Send a prompt to the AI provider asynchronously.
+ * @param session The AI session containing provider and model
+ * @param prompt The prompt to send
+ * @param response_cb Callback function for successful responses
+ * @param error_cb Callback function for error handling
+ * @param user_data User data to be passed to the callbacks
+ * @return TRUE if the request was successfully queued, FALSE otherwise
+ */
+gboolean ai_send_prompt(AISession* session, const gchar* prompt,
+                        ai_response_cb response_cb, ai_error_cb error_cb,
+                        gpointer user_data);
+
+/**
+ * Escape a string for JSON embedding.
+ * @param str The string to escape
+ * @return Newly allocated escaped string (caller must free)
+ */
+gchar* ai_json_escape(const gchar* str);
+
+#endif /* AI_CLIENT_H */