Handle disabled classes API

[Mahin2076]

What changed

• Validate that the My Classes API response is an array before rendering it.
• Show the API-provided disabled message for MYCLASSES_DISABLED/403 responses.
• Surface unexpected API responses through the existing error layout.
• Treat a missing teachers field as an empty list.

Why

When a district disables My Classes, the endpoint returns an error object instead of an array. The Classes view passed that object to .map(), producing TypeError: t.map is not a function and hiding the useful API error.

Impact

Users now see a clear administrator-disabled notice instead of a runtime crash. Valid class responses continue rendering normally. This does not bypass tenant restrictions or expose disabled schedule data.

Validation

• pnpm build
• git diff --check