Replace sparkle with app logo that's easier to replace

[pamelafox]

Purpose

I often get asked how to replace the logo, and I realized it's not straightforward, due to our current use of a "Sparkle" react component instead of an actual img tag.
This PR replaces the Sparkle with an image tag, so developers can replace the src.
The placeholder logo is the sparkle, however, so it will work the same as always.

Does this introduce a breaking change?

When developers merge from main and run the server, azd up, or azd deploy, will this produce an error?
If you're not sure, try it out on an old environment.

[ ] Yes
[X] No

Does this require changes to learn.microsoft.com docs?

This repository is referenced by this tutorial
which includes deployment, settings and usage instructions. If text or screenshot need to change in the tutorial,
check the box below and notify the tutorial author. A Microsoft employee can do this for you if you're an external contributor.

[ ] Yes
[X] No

Type of change

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[X] Refactoring (no functional changes, no api changes)
[ ] Documentation content changes
[ ] Other... Please describe:

Code quality checklist

See CONTRIBUTING.md for more details.

• The current tests all pass (python -m pytest).
• I added tests that prove my fix is effective or that my feature works
• I ran python -m pytest --cov to verify 100% coverage of added lines
• I ran python -m mypy to check for type errors
• I either used the pre-commit hooks or ran ruff and black manually on my code.

[pamelafox]

I removed the files and CSS rules because they're no longer used, these were removed in @zedhaque's PR a while ago that improved responsive design.