Summary

Fixes #2422.

The Overview page currently conflates three different Schema Registry states into the same null value:

• Schema Registry is not configured
• The overview fetch was skipped
• The overview fetch failed

That lets request failures or skipped requests render as Not configured, even when Schema Registry is configured and working.

This patch keeps the change frontend-only and narrows the Not configured fallback so it is only shown when the backend explicitly reports feature-not-configured semantics.

What changed

• add schemaRegistryError to the overview state
• request schema registry info unless permissions are explicitly resolved to canViewSchemas === false
• treat Code.Unimplemented as Not configured
• keep other schema-registry errors as errors instead of collapsing them into null
• show Unavailable in the Overview card when the schema-registry overview request fails

This intentionally does not change the existing UX for users whose schema permissions are explicitly denied.

Testing

• vitest --config=vitest.config.unit.mts src/state/backend-api.test.ts
• vitest --config=vitest.config.integration.mts src/components/pages/overview/overview.test.tsx