Add toolbar text insertion and tighten text edit cleanup

[rezailmi]

Summary

• add text as a canvas element kind with toolbar and T shortcut insertion
• create new text nodes as editable generated paragraphs with centered default text and frame-style placement
• start inline editing immediately after inserting text and delete generated text nodes when committed empty
• guard toolbar insertion against unsent comment drafts and clean up attached comments when generated text is removed
• extend toolbar, provider, and settings shortcut coverage for the new insertion flow and regressions

Testing

• bun run test
• bun run build