Add rich text document type and Tiptap editor support

[beNative]

Summary

• add a Tiptap-powered rich text editor and diff view and integrate it into the document editor when the new rich_text doc type is active
• extend document metadata, classification, exports, and repository flows to recognize the rich_text type and default HTML handling
• surface an editor engine toggle, keep Monaco functionality for source mode, and add the required dependencies

Testing

• npm run test:markdown

---

Codex Task