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/command/cmd_defs.c

@@ -2771,6 +2771,61 @@ static const struct cmd_t command_defs[] = {
               "/force-encryption policy block")
     },
 
+    { CMD_PREAMBLE("/ai",
+                   parse_args, 0, 5, NULL)
+      CMD_SUBFUNCS(
+              { "set", cmd_ai_set },
+              { "remove", cmd_ai_remove },
+              { "start", cmd_ai_start },
+              { "clear", cmd_ai_clear },
+              { "correct", cmd_ai_correct },
+              { "providers", cmd_ai_providers })
+      CMD_MAINFUNC(cmd_ai)
+      CMD_TAGS(
+              CMD_TAG_CHAT)
+      CMD_SYN(
+              "/ai",
+              "/ai set provider <name> <url>",
+              "/ai set token <provider> <token>",
+              "/ai set org <provider> <org_id>",
+              "/ai remove provider <name>",
+              "/ai providers",
+              "/ai providers-list",
+              "/ai start [<provider>/<model>]",
+              "/ai model <model>",
+              "/ai clear",
+              "/ai correct <message>")
+      CMD_DESC(
+              "Interact with AI models via OpenAI-compatible APIs. "
+              "Supports multiple providers (openai, perplexity, custom). "
+              "Each provider has its own API key and endpoint configuration. "
+              "Chat history is maintained per session and not persisted locally.")
+      CMD_ARGS(
+              { "", "Display current AI settings and configured providers" },
+              { "set provider <name> <url>", "Add or update a provider with custom API endpoint" },
+              { "set token <provider> <token>", "Set API token for a provider (e.g., openai, perplexity)" },
+              { "set org <provider> <org_id>", "Set organization ID for a provider (optional)" },
+              { "remove provider <name>", "Remove a custom provider" },
+              { "providers", "List all available providers" },
+              { "providers-list", "List configured providers with keys status" },
+              { "start [<provider>/<model>]", "Start new AI chat (e.g., openai/gpt-4o)" },
+              { "model <model>", "Change model in current chat window" },
+              { "clear", "Clear current chat history" },
+              { "correct <message>", "Correct last user message and get new response" })
+      CMD_EXAMPLES(
+              "/ai",
+              "/ai set token openai sk-xxx",
+              "/ai set token perplexity pplx-xxx",
+              "/ai set provider custom https://my-api.com/v1/chat/completions",
+              "/ai set org openai my-org-id",
+              "/ai remove provider custom",
+              "/ai start openai/gpt-4o",
+              "/ai start perplexity/sonar",
+              "/ai providers-list",
+              "/ai clear",
+              "/ai correct I meant something else")
+    },
+
     // NEXT-COMMAND (search helper)
 };