Add full-featured AIChat component

AIChat (elements/ai/AIChat) is a complete LLM chat UI built on new hooks:
- useAIChat: streaming conversation state (send, stop, regenerate,
  edit, feedback); `send` may return a string, async iterable,
  ReadableStream or fetch Response
- useAIChatHistory: saved conversations with optional localStorage
  persistence and generated titles, shown in AIChatSidebar
- useAIChatSpeech: read aloud via browser speech synthesis or a
  custom `speak` returning audio
- useAIChatDictation: voice input via browser recognition or a
  custom `transcribe`

Also adds a model picker, message editing, thumbs up/down, markdown
export, scoped markdown styles in base.css, the twuiReadTextStream,
twuiStripMarkdown and twuiGroupByRecency utils (with tests), and a
playground page. AIPromptBlock is kept and marked deprecated.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/elements/ai/AIPromptBlock.tsx

@@ -19,6 +19,10 @@ type Props = {
     placeholder?: string;
 };
 
+/**
+ * @deprecated Use `elements/ai/AIChat`, which manages the conversation,
+ * streaming, stop and regenerate itself.
+ */
 export default function AIPromptBlock({
     model,
     promptFn,