Add UI customization features

[yulmwu]

Refs: #18

[codecov]

Codecov Report

❌ Patch coverage is 77.59336% with 54 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/service/app_config_service.go	78.16%	21 Missing and 10 partials ⚠️
internal/http/handlers/handler.go	79.62%	8 Missing and 3 partials ⚠️
internal/repo/app_config_repo.go	77.77%	4 Missing and 4 partials ⚠️
internal/http/router.go	0.00%	4 Missing ⚠️

📢 Thoughts on this report? Let us know!

[yulmwu]

Codecov Report says there is missing coverage, but upon checking, it is negligible.
will merge the PR after adding additional customizable fields (keys) and refactoring.

[yulmwu]

This PR ends here. for more information, see #18 comments