feat: Intelligent-UI, interactive responses and versioned artifacts

[hsuanguo]

Summary

Add interactive, versioned answers directly to Pi Web conversations using an explicit pi-html Markdown fence.

Similar to https://openai.com/index/gpt-6-for-everyone/

Features

• Render completed assistant HTML/CSS/JavaScript answers as sandboxed interactive previews. Streaming output starts collapsed; ordinary HTML examples and user/tool content remain inert.
• Provide source switching, expansion, restart, copy, and standalone HTML download. Inline previews follow the chat column width and their natural content height.
• Show bounded, copyable runtime and promise error details with source-relative locations.
• Group optional stable artifact IDs into automatically numbered versions on the current conversation branch. Earlier occurrences stay collapsed, and older source is loaded on demand.
• Preserve history across reloads and include compacted or paged-out messages through a read-only branch index.
• Guide generation through Pi Web's built-in prompt without requiring a skill or changing global Pi configuration or extension APIs.
• Add English, Simplified Chinese, and Traditional Chinese UI strings and development notes.

Behavior Notes

Switching versions initializes the selected source and resets live inputs. Preview state is not persisted or migrated between versions. Downloaded HTML is the original standalone source; Pi Web's sandbox applies only to its embedded preview.