feat: add support for user apps

[braindigitalis]

Finishes off support for user apps. Adds a few interaction methods, and fixes the interaction json parse to not die.
Fom original PR by Ray21

Documentation and examples to go into docs once this reaches master. For a working example see src/userapptest/userapp.cpp

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

[netlify]

✅ Deploy Preview for dpp-dev ready!

Name	Link
🔨 Latest commit	c7928b0
🔍 Latest deploy log	https://app.netlify.com/sites/dpp-dev/deploys/6716a219c8fe5800084ea9dc
😎 Deploy Preview	https://deploy-preview-1304--dpp-dev.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify site configuration.