feat(ui): Allow to configure config refresh interval

[PythonGermany]

Summary

Adds config option to customize config refresh interval in the UI (refresh is used to get update announcements)

Checklist

• Tested and/or added tests to validate that the changes work as intended, if applicable.
• Updated documentation in README.md, if applicable.

[PythonGermany]

Not sure if that is whats expected. Like this it behaves the same as the endpoint interval option, there setting the value to zero does not make the config invalid but instead just uses the 1m default

[PythonGermany]

The issue could be solved in a similar way to how config bool values are handled:

gatus/config/ui/ui.go

Lines 118 to 120 in 1df0801

	if cfg.DarkMode == nil {
	cfg.DarkMode = &defaultDarkMode
	}

For a duration this would be:

• nil -> set default
• zero -> return error

Only question would whether to make a new pr where this is also "solved" for endpoint interval config and possible other occurrences. Changing this would only be a breaking change for invalid configurations.