Custom 404 page with lost dragon

[jimmeak]

Summary

• Add a custom Django 404 handler and public 404 template.
• Add a generated fantasy dragon PNG for the not-found page.
• Add responsive skin grid rules so the 404 page is readable on narrow screens.

Closes #251

Verification

• docker compose -f docker-compose.server_ci.yml run --rm web python3 manage.py check
• docker compose -f docker-compose.server_ci.yml run --rm web python3 manage.py test ddcz.tests.test_integration.test_404 ddcz.tests.test_integration.test_skins
• Selenium screenshot check in the repo CI Docker stack for desktop and mobile; confirmed the PNG loads and mobile has no horizontal overflow.

Note

The repository default branch is master; no main branch is configured on the GitHub remotes.

[Almad]

why not! :) you just have to mark it as non-draft for me to be able to merge it

[Almad]

I like, just please confirm you're the author of the dragon or that it's published under a permissive license / provide attribution