feat(view, viewmodel): display project demo

[huyenltnguyen]

Checklist:

• I have read and followed the contribution guidelines.
• I have read and followed the how to open a pull request guide.
• My pull request targets the main branch of the repo.
• I have tested these changes locally on my machine.

This PR:

• Adds solutions and demoType properties to Challenge class
• Updates the DescriptionView widget to conditionally display the demo button:
  • If demoType is null, just display the challenge description and instructions
  • If demoType is onClick, show "Build an app that is functionally similar to ..." with a text button that triggers the demo dialog
  • If demoType is onLoad, show "Here's a preview ..." with a text button that triggers the demo dialog - Currently blocked by Add solution code to the metadata of the first workshop step freeCodeCamp#61177
• Implements a ProjectDemo widget that displays the project demo
• Closes Show demo project on workshop and lab views #1564

Screenshots

Lab

Screen.Recording.2025-07-01.at.17.38.32.mov