fix(ui): handle undefined databases and customOptions in formatConfig

Summary

Fixes the Configuration page crash (infinite loading spinner) caused by TypeError: can't convert undefined to object.

• formatDatabases() and formatDumpCustomOptions() guard against null but receive undefined when the /admin/config response omits databases or customOptions fields
• This happens on virtually every fresh deployment — the default config template (config.example.logical_generic.yml) has databases commented out, so the backend projection omits it
• Replace incorrect as Type | null type assertions with ?? null (nullish coalescing) to convert undefined to null

Closes #697 (closed)