Emission Summary form: api should return data in the same shape as the schema expects

#### Describe the task

There is an unnecessary transformation step that requires the frontend components to know about the emission category types and how to break them down, effectively doing double duty with the backend logic.
The API returning the right shape of data, coupled with a flexible schema, should fix that

#### Acceptance Criteria

- [ ] first
- [ ] second
- [ ] third

#### Definition of Done

- [ ] first
- [ ] second
- [ ] third

#### Additional context

- Add any other context about the task here.
- Or here

No files, tests, or entry points are named. Trace the Emission Summary API response, the schema, and the frontend transformation that breaks down emission categories; done means the API returns the schema's expected shape and the frontend no longer performs that transformation, but the acceptance criteria still need definition.

Written by the indexing model from the issue text.