feat: Added file size limit to avatars and banners

[Jaskowicz1]

This PR adds a file size limit to avatars and banners as Discord seemed to silently introduce an avatar file limit (either that or it's always been there and it wasn't properly working the other day).

Code change checklist

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

[netlify[bot]]

✅ Deploy Preview for dpp-dev ready!

Name	Link
🔨 Latest commit	e5b02120a0d87e36782ac2096611dd857c81c14a
🔍 Latest deploy log	https://app.netlify.com/sites/dpp-dev/deploys/669e5f40a960a30008db5efb
😎 Deploy Preview	https://deploy-preview-1209--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.